The short answer is no in my opinion because Benadryl® is an antihistamine used in treating people and pets. It counteracts histamine which causes inflammation and itchiness whereas flea meds might poison a cat because they contain poisonous substances - active ingredients: Pyrethrin and Pyrethroids.
An adverse reaction by the cat to these toxins affects the cat's nervous system. This is a poisoning as opposed to simply an allergic reaction.
However, the adverse reaction to flea meds may also be an allergic reaction causing hives and itching and worse - even death.
I suppose Benadryl may alleviate the affects of the allergic reaction but there may other adverse reactions such as vomiting and diarrhoea and as mentioned nervous system damage.
The point is that if there is an allergic reaction by a cat to flea meds, simply administering Benadryl cannot be guaranteed to be a complete treatment and it would be far too risky to assume it was.
There is only one course of action: the vet.