A couple of observations here for random guy.
First, she sees you as Beta. If she perceived you to be Alpha and feared not having your around she'd never do this.
Second, because of #1 I think you two are seeing your relationship differently.
To you, she is your girlfriend, you're exclusive and this is a long term deal.
But to her, you are a just Beta chump, it is no longer a long term relationship and you are more friends than lovers.
We're watching the classic branch swing in action. For women, it's just too darned tough for them to get direct and end it. So the species master of indirect will play it out, taunt, deflect, minimize, etc until you spin out of control and do something that enables her to end it w/o guilt. We all know who she'll go to. In many ways, it will be you who drive her to him. Plus, she'll do all this crap purely on emotional whims and urges with no real premeditation - isn't that special.
Search the forum for "triangles" - she's just created one. For women (and men) who are really into the other party, triangles never cross their minds.
Bonus: We have V-day coming up. Do you really think she'll tell you "everything" he gave her and "everthing" she gave him.
Now, do you want advice on what to do?
I know SS gets wings connected in the different cities. Maybe we need to start another get together - basically the "It's Over" intervention. Get a bunch of the fellow SS'ers to meet the OP's of these threads and do what his father, buddies and friends have failed to do.