I have a  friend who for the last 3 months is experiencing erratic BP
readings from 220/180 on the hi side to 120/80 on the low side. High
readings are usually followed by a period of weakness lasting 1-2 days
and usually spent
sleeping. There seems to be no specific reason for the onset these
attacks. She is not taking drugs other than enaprine 

What are possible causes for this condition? What tests are
appropriate to diagnose cause? She has been placed on a Russian drug
called "enaprine" is this advisable?

The only way to know what is happening is to know the heart rate at
the time that the blood pressure was taken?then I could tell whether
the high bp was from tachycardia or something or constriction of the
peripheral blood vessels. Based on the drug that your friend is on,
I?m guessing that the problem is mainly from constriction of
peripheral arteries. I did a search for enaprine, and sure enough the
only page that came up was in Russian. Fortunately I know some
Russian, and Enaprine is the Russian trade name for enalapril, which
is an angiotensin converting enzyme inhibitor. ACE inhibitors such as
enalapril are very effective medicines with few side effects. It works
by stopping the release of a hormone that constricts blood vessels,
and lowering the strain on the heart. This is the preferred first-line
treatment for patients with left ventricular hypertrophy. These types
of drugs also enhance the effects of diuretics, but your friend is not
taking those, there?s no chance for interaction (diuretics are
contraindicated in diabetes?random side note). The only reason you?d
want to stop taking enalapril is if you experienced any signs of
angioedema (swelling and rash, itchiness). Perhaps the erratic bp is
from not complying with the schedule of the prescription? Make sure
she?s taking the drug as prescribed. This drug is given as 2.5-5 mg
doses at first, and after time up to 40 mg daily.

this is a very atypical situation.  blood pressure normally fluctuates
but not to this degree, therefore she needs workup for underlying
causes such as tumor of the adrenaline gland, narrowed kidney artery,
hormone endocrine disorder - the best person to see , now now now now
now   is a NEPHROLOGIST, who is the keeper of the blood pressure
realm.