I am a bit confused as to your question. Are you asking that if she takes over the counter Benadryl will this affect her test taking abilities?
If that is the case, technically the benadryl does not have any psychoactive effects as far as memory or related impairment in-so-much-as it will make her extraordinarily sleepy. Benadryl is primarily composed of an anti-histamine, which causes drowsiness--i.e. why you are asked not to operate machinery while on the drug.
I would not necessarily suggest taking the drug before an exam if she is not used to being active or thinking critically while under it--simply because it will make her less alert and could, for this reason, result in a lower score.