The amount of sexism or internalised misogyny on display in this thread and the last, including from the OP is worrying. At least the majority of posters agree that the GF in this scenario was the mature, sensible one after being strung along for so long by a future faker.
@OneGreenPoster your son is cruel to have used the GF as a placeholder. He belittled and dismissed her. I agree with others who said I think this was death by a thousand cuts for the GF.
Once she ended things you said he spoke to her the next day and offered to go buy a ring the day after that? Not that he suddenly realised that he was being ridiculous and that he really did want to marry her so why put it off, but begrudgingly hoping she'd accept the shut up pity ring so he might be able to hang onto his placeholder for a bit longer.
He didn't do the decent thing and look for a place to stay. I bet he hoped he could wear her down and get her to take him back. As a placeholder only of course.
She did not accept the shut up ring, and bloody good for her! So your son has been harassing her while she was away for a holiday! He harassed her so much that she had to ask him to move out immediately and block him!
Then instead of getting his stuff together and getting out of her property asap, he planned to ambush her when she returned home!
I know you're determined to paint the GF as in the wrong and your son as the poor hard done by party, but he is not the decent, sweet boy you think he is. He has acted terribly, and strayed into possibly abusive territory with the harassment and planned ambush.
He's very lucky your DH sees what he's really like, but you aren't helping him at all with your take on this.
There is apparently a large number of lonely, young single men these days. Women have realised their worth thank fuck! A lot of women are choosing to remain single these days because men, like your DS, are just not good enough. They are lazy, entitled, and not a little sexist, and these young women are seeing this and realising that actually they are much happier either single, or finding a partner who will treat them decently for starters, and wants the same things they want.
If your son doesn't buck up his ideas very quickly and learn the lessons his father is trying to teach him, he's going to end up one of those lonely men. He might pinball from relationship to relationship, but he won't be truly happy, he'll be with a string of someone's but lonely - like I assume your DH is truly happy to have met, married and had a family with you, being a team through life's ups and downs, working toward shared goals and caring about each other's needs - or me might just find himself alone, and lonely.
The GF on the other hand, it sounds like she's a smart young woman who is pro-active in going after what she wants! I like the sound of her, and I think she'll be very happy with her decision to dump your son, even if she's hurting now, and I expect she is hurting very much. She loved your son enough to want to build a life with him, and he belittled and demeaned her. She was right to dump him.