WWYD? NBR - a little long

Rach21

My sister is 23. Even though she's very pretty, funny, and smart, she's never been on a date, kissed, or anything. Where she works (her first job after college) she became good friends with 2 people, I'll call them Roxie and Dan. Both Roxie and Dan are married (to other people) but they liked to go to lunch and hang out with my sister at work. Well, Roxie and her family moved out of state. Dan continues to eat lunch with my sister, go on walks on their 20 minute breaks around the building, and texts/calls her outside of work. Wouldn't be so much of a problem except that a) he's in his forties, b) he and his wife are having marital trouble, and c) he's mentioned to Roxie and my sister that he and his wife have both been unfaithful to each other in their past. (Although, when I was single, I would never really hang around with married dudes.)

My mom has gently voiced her concerns about a married man contacting her. My sister accused my mom of being irrational, said they were just friends, and how he was texting her for advice since he and his wife got into a fight. I told her if my DH and I were hitting a rough patch, I'd be really upset if he called up a female friend to vent about it, advice or not. She insisted they were just friends, and I believe that she truly doesn't have any feelings for him outside of friendship.

Dan, on the other hand, seems to me like he's getting ready to put the moves on my sister. I really, really, really don't want my sis dragged into the middle of his relationship woes. I don't want my sister to be branded a homewrecker, though I feel like she's being very disrespectful to his wife by letting Dan badmouth his wife to her (no matter what kind of wife she is). I have a feeling that this whole situation is going to ruin my sister's reputation, if not her innocence.

I don't know if I should give some sisterly advice, or if I should hire the mob to rustle up this Dan guy, dunk his feet in wet cement, and throw him in the river. WWYD, bumpies???