In conditionals (and a few other situations), we can use a 'remnant' of subjunctive. We don't use subjunctive much at all in English anymore, but we used to, and this is one of the bits 'left over' from that time. Both "...if she WERE weak" and "....if she WAS weak" are correct (the writer/speaker chooses). 'Were' is better in a literary sense, but you'll generally find that older people use 'were' ("If I were rich....") and younger people use 'was' ("If I was rich..."). Choose which one you prefer to use yourself. Personally I use 'were', because I'm older.

I get confused myself about when to use "was" or "were" in unreal conditional (or similar) sentences . According to Grammerly such sentences express "events that are hypothetical or improbable." Grammerly gives an example similar to yours: "If I were a rich man, I would make more charitable contributions." (You could substitute "If he were a rich man...." to think about it in the third person.) https://www.grammarly.com/blog/conditional-sentences-was-instead-of-were/

I should have added: this is an example of a Second Conditional. A hypothetical situation. If + past plus would (or other modal) + bare infinitive If I had a car, I would drive to work.

This is the subjunctive: https://grammar.collinsdictionary.com/us/easy-learning/the-subjunctive