I can see why you are hurt for the ex, if your DD started dating him at 15, and they are now 20, he's been a part of your family for a long time. Anyone she dated at 15 will have a very different relationship with you as a man she'll date from now onwards, you are now welcoming a man into your home, rather than a boy you saw grow up, any new boyfriend isn't going to be the same for you and the rest of your family.
But, so very few people marry the person they dated at 15/16. Very few of my friends were single at 16, but now in our mid 30s, only one couple are still together.
It also might just be her getting the ex out of her system, long term relationships are hard to walk away from, particularly ones from such a young age - it's unlikely she has a completely separate friendship group, she's only ever been with him as an adult. Grabbing hold of the nearest person rather than have a go at being single (which is a bit scary) is very normal. She's only known being part of a couple. She's unlikely to have ever gone out on the pull with her friends, while she might have had blokes try to chat her up, she's never been in a position to do anything about it, much easier to go back into another relationship with someone she knows. (and he might not have the best reputation, but then, she knows him and has always known him as an adult, his track record is known to her, he isn't a new person to get to know.) You might also find their mutual friends are encouraging them to be together, she's the one in the nice stable relationship and has always been that one, she's not a single friend, they might be trying to get back to the status quo within the group, albeit with a different coupling.
Step back, be nice, he might be the love of her life. He might be a 'getting over the ex' relationship. Don't do anything that could damage your relationship with your DD until you know.