She's certainly dabbling with a moral grey area. However, he isn't her client and the friend isn't at work, she doesn't necessarily need to uphold professional boundaries in every single aspect of her personal life.
I have a friend who's husband made me uncomfortable with his 'harmless' sexualised comments and behaviour towards me, and he definitely tried (and failed) to spark up a close friendship with me when they were having difficulties in their marriage. She was aware of it, it wasn't just me but with almost all of her female friends. Everything I replied to him went through a filter of "How would my friend feel about my response?". However I didn't want to lose my friendship with her because of her dumbass husband, so I didn't get overly involved by telling her of every instance he did something that made me feel uncomfortable. I just wouldn't entertain it. On the other hand, if my friend looked at my 'conversations' with him, I would be absolutely perfectly happy for her to do so, and tbh I assumed she would. I'd never call her out for 'snooping'.
OP, this is all to say that your friend is in a difficult position BECAUSE OF YOUR HUSBAND and HE is the one who's behaviour is in question here. Although, it is not okay that she made a comment on you 'snooping' - that's none of her fucking business!
I suggest you talk to your friend about it honestly, she is in a difficult position with it too... ask her how SHE feels about it? She might drop you as a friend if you make her feel like she's responsible for this. Edit: particularly if she's going through a hard time herself, and just wants a friend, but he's sabotaging it by objectifying her and sexualising it.
But if she seems to want to protect hers and your husbands 'friendship' over hers with you, then cut her off. Any true friend would say "I don't give a toss about a friendship with your husband, I'm your friend, not his..." Etc.