In Thomas Moore's book Soulmates, p. 208 (1994), Moore has this quote,
apparently from Marsilio Ficino, about Pythagoras:
  "Pythagoras asks that we not let a friend go lightly, for whatever
reason.  Instead, we should stay with a friend as long as we can,
until we're compelled to abandon him completely against our will. 
It's a serious thing to toss away money, but to cst aside a person is
even more serious.  Nothing in human life is more rarely found,
nothing more dearly possessed.  No loss is more chilling or more
dangerous than that of a friend."  Exactly where did Marsilio Ficino
say this?  Apparently Pythagoras did not do any writing of his own.  I
need as close to a primary source as possible, so I guess Ficino is
good enough, unless there is a better, more primary source.  Is this a
job for Scriptor?

Hello Sylvia,

The quotation comes from a letter that Ficino wrote to Andrea Cambini.

Here is the quotation in context in Volume 2 of The Letters of
Marsilio Ficino (Shepheard-Walwyn Ltd, 1978):

"Marsilio Ficino to Andrea Cambini, an excellent fellow citizen.
Now what is pitiable in love? For it not to be requited. What is
impious? To love on account of oneself what should be cherished for
the sake of another; and contrarily, to cherish for the sake of
something other, that which should be loved for its own sake. What is
most disgraceful? To hate someone you have loved. In this respect we
are much at fault, for although we do not abandon other things,
however trifling, unless compelled to do so for a serious cause, yet
we forsake and abuse a friend, a priceless treasure, led into it by
any cause, often a very light one.
 Pythagoras commands that we should not lightly discard a friend,
whatever the cause; rather we should bear with him for as long as we
are able; and we are able until we are compelled to leave him wholly
against our will. To throw away money appears to be serious indeed,
but to cast aside a man is most serious. Nothing in human life is more
rarely acquired, or more dearly possessed. No loss bodes more ill or
is more perilous than that of a friend."
source:
Letter 51, Page 63, Volume 2 (being a translation of Liber III)
The Letters of Marsilio Ficino (Spepheard-Walwyn Ltd, 1978)
Available for purchase from Abebooks:
http://dogbert.abebooks.com/abe/BookDetails?bi=99130720

The translators of Letter 51 have placed a footnote immediately after
the phrase, "Pythagoras commands that we should not lightly discard a
friend."
The footnote cites:
Iamblichus, Life of Pythagoras, Chapters XXII & XXXIII.

Here are the passages to which the translators are referring in
Iamblichus' Life of Pythagoras:

"And again they say, that friendship is not to be rejected on account
of misfortune, or any other imbecility which happens to human life;
but that the only laudable rejection of a friend and of friendship, is
that which takes place through great and incurable vice."
Chapter XXII, p. 55.
"Again, according to them, friendship should not be abandoned on
account of misfortune, or any other imbecility to which human life is
incident; but they said, the only approvable rejection of a friend and
friendship, is that which arises from great and incorrigible vice."
source:
Chapter XXXIII, p. 119.
Iamblichus, Life of Pythagoras, 
translated from the Greek by Thomas Taylor (John M. Watkins,
Publisher, 1965 reprint of 1818 first edition).
The same edition is available from Abebooks:
http://dogbert.abebooks.com/servlet/BookDetailsPL?bi=172461166
Some less expensive paperback editions are also available:
http://dogbert.abebooks.com/servlet/BookDetailsPL?bi=232271702

search strategy:
I looked through the writings of Ficino at a local library until I
found the quotation. I then followed up with Iamblichus' "Life of
Pythagoras."

If anything is unclear, please let me know via the "request
clarification" feature.
Thanks again, Sylvia, for the interesting questions!

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Request for Answer Clarification by sylk-ga on 19 Oct 2003 06:19 PDT

Dear juggler and scriptor,
   Your answers are truly extraordinary.  I have been telling many
people about the scholarly researchers on Google's staff.  Thank you
so much for the unbelievable job.  The answer now seems perfect
(obviously I will have to verify the source, but you guys have always
been thorough in your documentation.)  Juggler, thank you for pitching
in on this one.  Scriptor, you have not disappointed me. Your answers
always teach me and enrich my knowledge tremendously.  As far as I am
concerned, you guys are both worthy of a five star rating and Google
is blessed to have you both.
Thank you so much,
Sylvia

---

Clarification of Answer by juggler-ga on 19 Oct 2003 11:19 PDT

Dear Sylvia,
Thank you so much for your kindness and generosity. Scriptor and I
have arranged a split of the tip.  Thanks again!
-juggler

This was a very difficult question, and I am grateful to both Scriptor
and Juggler for their help.  Please split the tip since both juggler
and scriptor contributed considerable information on this one. I hope
this is possible to do fairly.
Thank you,
Sylvia

Dear Sylvia,

I am glad to see that your question got a truly fantastic answer by my
colleague Juggler now. I would like to thank you for your generous and
encouraging offer to tip me for my previous work, although it did not
lead to satisfying results, but unfortunately there is no way to leave
me a tip under these circumstances. And I must admit that accepting
such an amount for delivering less-than-perfect work would leave me
conscience-stricken.
I hope that I can grind out this embarassing failure at a future
occasion. You have been one of the most wonderful and friendliest
customers I met duing my work as a Google Answers Researcher, and it
makes me very sad that I have disappointed you.

Very best regards,
Scriptor

Dear Sylvia,

Like my colleague Juggler, I too want to express my gratitude. Thank you very much!

Sincerely,
Scriptor