I think that people can have more than one relationship at a time, as long as they're honest with each other. Letting her know about the guy sneaking could potentially break them up, but ask her to think about how she feels about open relationships before she goes off and dumps his cheating ass.
A better idea actually, might be to approach the guy, and ask him to come clean to his girlfriend, and tell her that it was his bad to do such a thing without her approval.
Let's say that two people have been together for four years (just a random number). Things are starting to get a bit boring between them. The dates aren't as interesting, the sex is dry (sorry for that), and they just feel like things aren't going anywhere.
Solution 1: Promise to stay friends, but break up as BF & GF.
Solution 2: Each go out and start having on the side affairs. At the end of the day this could make their relationship with each other more exciting.
Think of it this way: A lot of guys suffer from a condition which makes them have to prove that their bigger and better. So if the GF has recently gone on a great date with her side-dish lover then her main course lover will feel the need to prove himself.
Or maybe he'll be cool with it, and understand that their are just some things that he can't give her.
For me it would depend on how good I knew the parties. If the person being lied to was a good friend, I probably would tell them. But often, especially if they aren't in my inner circle I will try to keep myself out of the drama, and not be the catalyst, simply because I've sen what happens when that happens, and it's a mess for everyone involved.
Find out if the girlfriend is allowing this sorta thing to be going on? And yes, as said talk to the boyfriend first before saying anything... Secrecy will just make the end result of anyone's relationship involved a big mess, tho you could be the one getting some of the blame for saying the ugly truth~ But hey, someone has to clean the sh*t, right?!