When a friend invites your husband into a sexual encounter, they are initiating or participating in something which could destroy your marriage. Aside from not having any feelings for my friends' husbands, I wouldn't do anything which would jeopardise their marriage and family.
The man, of course, plays the biggest role in betraying his wife (and children, in effect) and would be the greater offender. The man would probably have had some kind of extra marital relationship at some point, with someone, if he's the type, or maybe not. The point is, for someone in your friendship group to orchestrate a context where something could happen that would lead to your marriage ending, and all which that ensues, absolutely involves the OP.
I don't think the OP would I'm any way let herself down by saying something because it would be her having to pick up the pieces of this situation, had it happened. This friend would have been part of an activity that may have torn OP's life apart. For someone to actively go after that, to deliberately go down that road, knowing the potential consequences is dreadful. Saying nothing will just make the friend think she got away with it.
I would just say 'DP told me that you propositioned him and I'm so hurt and disappointed that you would do something so low. There's no way that I can continue our friendship after this'.
And leave it at that. There's no need for witty retorts. Say it like it is, in private and let that be the end of it.