<human>: Can you succinctly summarize the text in 1-3 sentences?

My best friend has been going through a tough spot with his girlfriend recently. They've been growing apart but I know he still has feelings for her - which is why he hasn't broken up with her. The relationship is just grown a bit stale. They've been bf and gf for 4 years now. Just tonight, though, I got a facebook from her saying that if it wasn't for best friend that we would "... so be doing it right now".
This has extra weight on it though because we used to go out for a while. But there's been nothing between us for years and no hint of any chemistry between us. Our relationship was a "puppy love" affair that happened when we were in high school. (we're nearly finished college now). And their relationship has been a really serious one. Their first real serious relationship.
Our friends haven't even mentioned the fact we used to date in about 3 years so I know it's not a "frape" or anything like that. It just doesn't make sense to even bring it up. I'm worried it's serious because although no chemistry has been happening, I have been nicer to her on nights out. I walked her home when her boyfriend went off on a drunken walk and didn't look like he was coming back for a while and we got to talking for the first time alone in years.
I have absolutely no intention of starting anything with this girl. I have no latent feelings for her and my friend isn't a bad guy. He's one of the nicest guys you'll ever meet. I don't want to be the one who pushes these two apart either. But because he's my best friend, am I honor bound to tell him? Is it in the bro code? And how do I go about telling him if I do?
It was only the one comment because I decided to ignore it altogether. I didn't want any more information than had already been parted. If the relationship is at it's end, that's fine. If I tell him I might be the one providing the "out" for my friend if the relationship is truly over. But do I want to provide that out?
What do you think?
<bot>: my ex, who's going out with my best friend, just made a pass at me. Do I tell my best friend or ignore it?