I have a 20 year old daughter. Rather than speculating on what a "promise ring" means or represents or indicates about their relationship being "controlling", I'd just ask her.
If your daughter wants to wear it, then she should wear it. If she doesn't want to wear it, she shouldn't wear it. If she wants to wear it on an alternative finger and announce that it's not a promise to be married at all, then she can do that. Her hand, her ring, her choice. (Presumably what it means is something she will have discussed with the man who gave it to her.)
For me, this would be a non-issue. I also wouldn't try to tell her she should "expand her relationships" while she's away at college (does your husband mean he thinks she should break up with the boyfriend and date other men?). Again, how committed she wants to be to her current love is entirely up to her.
To me, it sounds like the two of them are trying to find a way to make a long distance relationship work. And maybe it will, and maybe it won't. I wouldn't try to influence it either way.