3) Women are also the masters of self-sabotage, especially immature ones which I'm gathering from OP. Just when things are "going well" they pull back or whatever. Some sort of defense mechanism against getting hurt. I've run into a few of these over the years and they are 110% a f*ucking waste of time. They are never happy and jump from guy to guy looking for something they'll never find. Then, when SMV plummets, they latch onto usually an abuser, addict, etc.
So, maybe his gf has met someone else and maybe not. Women don't NEED a reason to act like this.
It may just be another emotionally f*ucked up woman.
Go 100% NC. Then, if/when she reaches out, tell her you're been really busy and "thinking" a lot and leave it at that. Get her hamster wheel spinning. Then, plan your exit strategy. Remember, it's ALWAYS better to be the dumper than the dumpee so you may want to plan a preemptive breakup--you breaking up with her. Every time I've done this they've eventually reached out and wanted to make up, but by then I was over them.