I replied in a comment to your question in this post. It is able to be possible/ She is able to have drunk. To expand upon this:
She isn't able describes her current [in]ability.
to have drunk that much coffee describes a past action.
In this context, a current ability cannot describe a past action.
Your choices are
- Expressing a fact/belief/opinion
A: “Look! Twenty empty coffee cups! This is a mystery! She isn't able to (can’t) drink [i.e. currently] that much coffee.” Or
A: “The coffee machine is empty and Maria has disappeared. She wasn't able to (couldn’t) have drunk that much coffee, [i.e. at some time in the past] therefore some other people must have been here.” Or
- Expressing a probability/possibility
A: “That litre carafe of coffee is empty! I know that the cat likes coffee, and I left her alone for five minutes, but she wouldn't have been able to drink (couldn't have drunk) that much coffee.” [i.e. in the time allowed and because of its size] (The cat is assumed to be alive) Or
A: “I don’t think that it was coffee that killed her. Maria drank a lot of coffee every day but she wouldn't have been able to (couldn't) have drunk that much coffee.”
(You have also introduced "if" clauses - these have their own guidance: you should research "if conditionals".)