Generic may be the way to play this.
If she gets specific he can keep coming up with counter arguments and she'll find herself fighting from a negative position.
Realistically she doesn't have to prove her un-faith to him. Each individual is responsible before God for themselves.
Perhaps she could start by saying (if it's true) that her faith in God and Jesus ransom sacrifice is as strong as ever, in fact stronger even. She could follow that up with that she has found some doctrines that made her feel uncomfortable and upon further research and study she realized that some things that the society is dogmatic about - well, they shouldn't be.
By putting the blame squarely on the dogmatism of the society she can take the pressure off of discussing several doctrinal errors for which he may be more prepared than her.
However,
If he just pushes the point and wants to discuss specific doctrinal issues; she should be prepared with two or three that she's bothered by and prepared to discuss.
Just thinking about her situation is stressing me out!!