This is probably more common than we realize and I have to admit that I somewhat question the stern attitude that we, as a profession, take on things like this. People are sexual beings and money is a powerful motivator.
We could conjecture all day long about why the spouse gave her money, if he really gave her money, should he have given it if he gave it.
Really, what it comes down to is that it was his to give if he wanted to and really did give it. What she did to earn it - well, that's another topic.
I'd say that we need to be 1000% pure in our relationships with patients and family members. We need to be able to live with ourselves and with our God and realize that our reputation is easily destroyed, almost never repairable (if that's a word).
Without solid proof, leave it alone. If you feel the need to speak to her about it, I guess you will find the time and the way to do so but be very, very careful. She will not likely respond positively unless approached in a totally non-judgemental way. She will have to be the one to realize her error and admit it and that's just not very likely to happen, especially if you jump on her with both feet.
I'm sorry you lost a patient with whom you really connected. That is painful but time will help heal the wound.
As for what goes around comes around - maybe it does but somehow I never get to see it when it gets here.
Don't worry, everything will work out. If she really is concentrating more on money and relationships than on patient care, someday she will encounter a boss who lowers the boom on her. Although in honesty I must say I see nothing wrong with wanting to marry a man with money (doctor). Must we all aspire to poverty? Is there really anything wrong with meeting a mate where we work? This happens all the time, too. Should it be with a married patient or patient's spouse? No. But we all have heard of cases where the staff fall in love with a patient and they get hitched and are happy together. What is so unethical about that?