From the point of view of discussing it with his wife… that too is a highly personal thing. From my 24 years of marriage (I’m male), I would say that being:
- confrontational and
- focused on doctrine
are likely to be major fails. If they’re using NFP, I’m sure his wife doesn’t need a lecture on doctrine, she probably knows it well enough. Either a need isn’t being met, or a weakness is involved. We all have weaknesses, and someone shouldn’t have a guilt trip laid on them for having one just because it happens to be sexual.
This needs to be approached in a loving, non-accusatory, non-confrontational and non-emotionally-charged manner.
Some gentle inquiries about whether everything’s alright, is she stressed, can he help her with anything? All without mentioning masturbation. For some, masturbation can be a stress release. Perhaps abstaining is very frustrating to her, and with other daily frustrations, the glass overflows. We all have different capacities.
Suffices to say that the OP should really be taking this up with a priest, or perhaps a good Catholic-friendly marriage counselor or psychologist.
No need for us (not pointing the finger at you personally) to overreact with “her soul is in danger…” etc. Fact is all of our souls are in danger over something or other. I’m willing to bet that a good priest will be far more pastoral than we are!