I do not think it can be solved unless the man steps up to the plate. He has to explain to his now wife that he has this friendship that existed long before her. He must include her in the relationship and many times that puts the wife at ease.
My hubby has a dear friend and they do some stuff together now and then but they always ask me to join in. Sometimes I do and sometimes I don't.
I was smart enough to figure out that if he wanted to be with her he would have done so many years ago before he met me.