Been there - done that. Ín our case it's six of one, half a dozen of the other. Meaning, if your daughter is in for destructive relationships - then so be it - there is nothing you can do. Save your energy.
We've struggled with our daughter's, older, mentally controlling, gambling, violent and con artist boyfriend for years. He started off wowing her, buying her expensive gifts then throwing the cost of them in her face, making her fall in love with him and creating strains in her relationship with us. He made her relive her childhood, found a few issues in the past, and then made her fight with us over them. First time I met him he was eyeing other girls up and down. However, he expected us to fork out for expensive meals for them, (saying it's payback for when he pays for her meals at his apartment) wanted us to pay for her vacation with him & make his financial situation a little easier, he wanted a down payment for a condo (she doesn't have any money but on early dates asked her if we had any money saved for her and very interested in our financial situation). He constantly belittled her, makes her cry a lot, she is depressed when she is with him and when she is not with him, he called her dumb (she's a University student), but she never moved in with him as she couldn't afford to pay 1/2 the rent (and we won't pay it) she has told us stories of his past negative relationships, where and how he tried to extort money from his past girls. All negatives. She is the IDIOT. They've broken up several times and they keep getting back together. When she is not interested he keeps texting into the wee hours of the morning, threatens everything and anything and himself. We've called the cops because we had concerns for her safety once but they couldn't do anything unless there was an incident. Believe me, we've tried everything, spoken to doctors, sent her for counselling, nothing is helping. I believe she is now seeing him again and not telling us and they are thinking of getting married. No, we are not paying for the wedding, we are not attending, we are not buying them a wedding gift. She is on her own, penniless. Let him support her. We are not interested if they have any kids either. Yep, we've turned on our daughter too. I never thought I could have created such a stupid girl. It's all in black and white and she can't see it. I've been really stressed out by this drama and the only way to make it stop is pretend she doesn't exist.