I have a friend who's now in Spain and asks me what he can do to
amplify a weak wireless signal to his laptop. Alternatives?

Is this a general problem or location specific?

You can use something called a "rang expander" which essentially
"reads" the signal and then "repeats" it. This will amplify the signal
if placed midway between the source of the signal and the receiving
wireless adapter (i.e. laptop).

Linksys makes one: http://www.linksys.com/servlet/Satellite?c=L_Product_C2&childpagename=US%2FLayout&cid=1130267578138&pagename=Linksys%2FCommon%2FVisitorWrapper
or I'm sure you can find others on the internet.

You can also replace the antenna(s) on the wireless access point (the
one broadcasting the signal) to a "high-gain" antenna usually
available from the vendor of the access point or router.