Our approach is totally different. We are in this together. We are partners. He can search through anything of mine and I can go through his stuff if we so desired. It would be extremely rare if we did but we have that understanding.
That is how OUR marriage works. Maybe the op has a similar arrangements.
Anyhow the fact he is spending the op families money on something that is causing her pain, is enough reason to ask him to stop.
Any action we do as part of partnership shouldn't deliberately hurt partner. If one partner stands up and says this is hurting me the other partner must take that seriously.