E.:
It's not just FB that can be blamed - it's technology in general. There is Myspace, Classmates.com, LinkedIn (yes, it's a professional site but people find each other there!!).
And to top it off - it's not the technology - it's the relationship. If someone isn't feeling valued and appreciated they will seek it elsewhere. Especially if they expect their SO, spouse, BF, GF to read their mind and KNOW what they wanted, expected. This is why communication IS KEY to the survival of a relationship.
No, I haven't known anyone who's relationship ended because of FB or any other technology. I have had people find the gapping holes in their relationships because they got comfortable and in a routine - taking each other for granted and forgot to wine and dine their spouse...and they got attention elsewhere. NO, I don't condone it. And I won't be their scapegoat so they can go see someone either.
What I think will be funny - is that that "old flame"? the problems that broke them up all those years ago will still be there. They will most likely end up breaking up when the grass isn't as green as they thought it was. Hopefully your friend will learn from this!!
I would tell my friend I'm sorry.