when to step in

Sounds all to familar...as with my daughter who is now older and wiser, but after so many yrs of mental abuse is now suffering panic attacks and going grey we are sure through stress! we think her ex partner being a control freak to put it mildly! (yes that is the only word I can describe him as)! and won't admit it....hope your daughter manages to keep going and rise above things..

If he is being nasty in front of you, he may be nastier without an audience. This behaviour pattern is not uncommon but in your shoes I would be concerned.
I had a neighbour for a while (we were renting an apartment) - a UK of Indian heritage woman who had married someone of a different Indian religion. He was behaving very possessively and she was getting anxious - so much so she approached a stranger. Last time i saw her I said, very directly "This is emotional abuse. I think you should tell your parents even if you fear their response. The still love you and won't want you to suffer." I never managed to engineer another opportunity to talk to her, and i often wonder what happened to her.
I agree with Nsube. Your daughter is still a minor (?) and you need to express your concern about whether she is happy and reassure her she has your support.

This is quite worrying. On the one hand he may lose her, alternatively the controlling behaviour may worsen and she could find it increasingly difficult to resist or to break free.
It's hard to give advice as it may be seen as interfering, but your daughter needs to know that if things get worse she has an escape route with you. Don't overtly criticise though, or you risk alienating your daughter and that is the last thing you want.

This is what happened with one of my daughters - he lives separately now and shares the care of their two boys, who adore him and he adores them He was so controlling and nowadays can be downright nasty to her because he is bitter about them parting, which proves she was right to part from him when she did, as he won't attempt to control his behaviour towards her, even in front of their small children. Morbid jealousy is a horrible thing, and if men can't take the trouble to deal with it they have to deal with the consequences.

thanks for all the good advice i agree with you annobel that he is only young himself and has done an excellent job as a dad and has never negleted his responsiilities to my daughter or their child as has she i help out a lot with babysitting as i recognise their need for time together especially as they are both still young i have always taken an unbiased view but lately things have taken a new twist and he is becoming more and more controlling i fear as carol says that she will become resentful and by trying to hold on too tight he will loose her my sadness is that i see her becoming more and more down and that she is trying so hard to keep him happy at a loss to her own happiness

Are there some activities they could share? What you could do without being accused of interference, is volunteer to babysit while they go out together. If they were able to do this, he might not feel so threatened by her friends. He may be five years older, but this still makes him relatively immature. He has also had to shoulder responsibility quite early in life and probably feels insecure.

I agree with Carol. Perhaps your daughter's partner feels threatened by the fact that her friends, both male and female, are a bit younger than him. The fewer choices she makes for herself, the more controlling he may become, so she does need to start stretching her horizons a bit before this happens. If you have a good relationship with him, and if things don't get any better, perhaps a 'comfortable chat' - with no judgemental comments made - may help?

Unfortunately, if he doesn't get to grips with his need for control he will lose her. She doesn't need this sort of behaviour and will resent him more as time goes by. So many young women stick it out and put up with this behaviour until they start to realise they can make their own way. He is probably frightened that college will give her that opportunity. They could try Relate to see whether they can overcome this problem. If he doesn't collaborate with her, though, she will soon find that she does need you and her friends to get involved.