I went through that enough times that I eventually came up with a plan B and sometimes even a plan C.

Every girl I ever cryed over often didn't deserve that consideration at the end of the day. After I had my heart stomped on enough times I started to limit my exposure by having other options. Not being able to swim, I decided that being thrown from the boat in the middle of the stream was no better than jumping overboard and try to swim to shore. Either way I was gonna go under and not be seen again for a while.

So, I kept my options open, stayed with the better ones and moved on if they weren't working out. Once I left the usual problems behind and focused more on who I was with, things went better. However, I was well into my late 30's by this time and had some expirience behind me while 20 year olds have a way of taking everything to heart.

You don't own who you are with. They may stay with you for a variety of reasons and every relationship will be different from the next. 9 months is long enough to get hooked and betrayal is a hard thing to deal with.

I once asked a gir out that a buddy of mine was dating. No real relationship to speak of and he told me he wasn't interested in her. We went to a movie - no big deal. I knew she wasn't interested in me but I'm not sure why she agreed to the date. I wasn't out of her door and in my car before she called my buddy. When I got home an hour later he called me and made it very clear that you never fish from your buddies dock. Years later when a long term girlfriend made a play for him he turned her down - just like a real buddy should do.
I was told she did it to shake me up - it did. She wanted to get married but I never trusted her after that and 4 months later we split up. What she did was secondary to how she did it. After she had control of everything ( for a while ) I saw a complete change in attitude - demanding, critical, uncommitted, uncaring. All from a person I knew 4 1/2 years who was anything like those things. Once a woman cools off, she rarely ever warms back up to you.

Your son's ex already moved on a while ago and long before this happened. I take it she told him what happened over the phone and that she didn't want to go out with him any more? She could have skipped the details but she needed this diversion to make herself actually split up with him. She likely didn't want to go any further in the relationship but didn't really want to swim to shore either. Odds are this new deal won't last very long.
Tell him he's lucky to be rid of her. He won't feel that way right now - that will come later.