In my sitch, I dated and lived with my FWH for 7 years before we got married. During this time, he was completely faithful. If he had cheated on me during that time, I would never have married him, period.
Prior to us getting married, my FWW and I dated for about 5~7 years (too lazy at the moment to go check the calender, LOL). Much of that time we spent apart, as we attended separate universities. Somewhere in the middle of this, about two years before we were married, she "cheated" on me for about a month or two with another student at her university, someone who first became a friend, then took it further (sound familiar, doesn't it?). I was devastated, but after she told me that she never meant to hurt me, she was very, very sorry for what she did, she always wanted to be with me, she loved me more than anything, and she'd never do it again, I agreed to continue our relationship. We actually ended more in love with each other than we were before, perhaps because of all the sharing we did after she disclosed her A.
Unfortunately, something went wrong in our M, and here we are again, but this time it's different - her A was longer, she was more emotionally involved, and on disclosure her comments to me were basically along the lines of that I could take it or leave it, that it was up to me if we were to stay M'd, but she wasn't pushing for it either way. There was little remorse expressed for what she'd done, only a polite apology for hurting me in the process. She said she did it because it made her feel good, and she was tired of worrying about other people's feelings. So of course our recovery now is a lot harder.
My advise to "In_Distress" is, if he feels his GF is TRUELY remorseful about what she's done and she really wants to spend her life with him, then it's worth giving it a chance. Just don't make the mistake of relying on the vows of marriage to allow complacency to slip into how he treats her, and vice-versa. Also, she's already given signs of her weakness when someone else pushes boundaries, and she needs to do what needs to be done to let him feel that she will protect him from this in the future; her simply saying "I love you more than anything", and "I'll never do it again" is NOT enough, as my own experience has shown me.