I have to disagree with your friend when she said, "men and women can not be best friends". As of now I've had 3-4 male best friends throughout my life, and they were -so close- to me that I didn't want to bring romance into the picture
Now, as for your situation...
Don't get involved with his friends! I attempted that once, and regretted it (never been married, so he was just a bf of mine). We broke up, and soon after I starting being linked with his friend, and started considering a relationship...
Well, he found out, and freaked. out. Perhaps not all guys would have such an extreme reaction, but he was very hurt and upset at both of us... because imagine, if that guy STAYED as a best friend to your hubby, it would always interfere with their friendship. They would never be the same, I can assure you of that. To see your friend dating your ex-wife, whether you loved her or not, is almost a blow in the face... it's a "you couldn't keep her (for whatever reason), so I have her instead" message always being tossed around.
So... no, too much trouble there :/ It's not worth it, unless they completely separate, and you intend to break off some ties (or loosen then dramatically) with this "hubby" of yours.
Again, I can't say my own example speaks for everyone... but it's something I wouldn't even risk again