There are a couple of issues at play here. I'm going to address the most important info first.
There is such a thing as someone being misunderstood, making mistakes, having a hard life, etc. Whether of not you want to still accept a person in these circumstances is totally up to you and sometimes they need some sort of acceptance, so I commend you for treating him good and welcoming him into your home. With that being said, he has already shown agressive behavior to your daughter, by going to the party, insisting that she talk to him and then saying ugly things to her. His constant calling, trying to get her to answer the phone and/or call him is aggressive as well (his father's message shows where he may have learned the behavior to begin with). This behavior is something to be concerned about and to talk to Michelle seriously about. Some women will find this type of behavior flattering (especially younger women) not realizing that this behavior usually escalates over time. She may consider him a friend, but he's not looking on her as a friend, but as something to be possesed or something that belongs to him, and it can only get worse as time goes on. In fact, if she decides to split with him, I can almost guarentee she will wind up having to get a restraining order.
Let Michelle know that it's nothing to do with him being a 'low life' because you allowed him into your home, you accepted his presense without question which is more than what some parents would have done. Now he has crossed the line, his family is harassing her, he is harassing her and for the time being you don't want him in the house. She can still be friends with him, she just can't have him at the house.
Althought the girls are your daughters, they are no longer children. You have no obligation to allow them to stay with you, no less any of their friends. You are making the decision to allow the girls there and their friends, and it is your position to decide if someone should lose that privledge. It's not something that should really fall on Dave, and if you take the lead, it doesn't have to.
Let me know if you want to talk more about it.