Syag Lchochma,
If you want to discuss it respectfully as a concept, then ask how we can be sure someone is secure in their frumkeit if they are new and being considered for marriage.
This is absolutely the way to go about this discussion.
These are my thoughts on the question.
1. Is he/she well integrated into a frum community?
2. Does he/she have an open relationship with a rav, and feel comfortable bringing shailos to a rav?
3. Can he/she separate Judaism from individual Jews, so that cv’s an encounter with a rude frum person doesn’t shake his/her beliefs to the core?
4. Does he/she have frum friends, or does he/she interact primarily with non-frum friends?
5. Is he/she comfortable with saying “I don’t know, I need to ask”?
If the answer is yes to these questions, then I don’t think there is any more “fry out” risk with the BT then there would be with anyone else.