Do guys do this?

I think that the honest thing to do would be to either tell her I don't know and keep out of it, or give her some suggestions or advice that you really do think would help, but only if you have had some experience have some inside information about her situation. I don't think it'd be right to lie to her to purposely to steer her away from her relationship. That's what I'd do. That said, I'd add after trying to help telling her something like, "Let me know how that works out because I like how you asked me for help." Not knowing how well you know this girl, just say something that will leave the door open to talk more and don't be shy about telling her you like her too. Learn not to be shy in teling a girl you like her if you do. Otherwise she'll have no clue and maybe she also likes you. Might as well get it out.

I've actually been in this situation before (as the "friend"). I didn't, and wouldn't, attempt to sabotage the situation. To me that's being dishonest to her, and it'd be (in the end) hurting someone for personal gain. I'd just give my honest and hopefully helpful advice. If I thought the guy was being a jerk and such, I'd let her know. If I thought he was worth her time and the relationship could be salvaged and such, again I'd let her know. Honesty is the best policy.

It may hurt "losing" her (for lack of a better term) but the friend would be a better person for it.

I wouldn't TRY to do it but I wouldn't sweeten up anything I have to say to make them stay together. Seeing as how she is my friend I would tell her how I see it and not try to just bypass the obvious.

I personally would try and help her keep the relationship going, but I'd be hoping it failed. I wouldn't ever try and sabotage a relationship.