What can I do?

It's cruel of your D to use your GC as emotional blackmail Loubee but in your position, I'd bite the bullet for now and say you'll see your D and GC while her boy friend is there.

It wont be easy but at least you'll retain your relationship with your GC and be able to keep an eye on them both.

I don’t think you will have much choice Loubee. It’s a difficult and painful situation for you but for your gc and dd sake try to stay in contact with them. It’s a new relationship so coukd break down so keep communicating.

If you feel your gc is at any risk from this new relationship do not hesitate to contact social services or nspcc with your concerns.

If your gc is at nursery or school hopefully they will be another watching eye.

Lots of red flags here already.
Keep the contact going, keep your eyes and ears open and don't pass comment.
In addition to silverlining48's advice about contacting social services and the NSPCC, let your grandchild's nursery or school know your concerns now, in confidence.
They can indeed be watchful.

Maintain contact, but does she mean she will continue to stay with you for three to four nights but boyfriend, who is already abusing her, must come too? Why?
A dangerous situation; be very careful.

This is very worrying for you Loubee but you need to keep a close eye on this man and your D and GC. If you are around you will be able to tell if things don't seem right and contact the safeguarding team at social services if you are worried.

Thank you all for your replies.

I met him for literally an hour and every time she spoke he cut her off and belittled her. I was seeing my gc as usual up until a fortnight ago, I share childcare with her due to work and college and she started saying that she can get them from school etc when I asked her if she was no longer in college and she told me she had quit. I’ve seen gc once in last fortnight, they are as sad as me and keeps asking if I’m still best friends with them - it’s breaking my heart

I second and support every single one of the comments here saying keep in contact as much as possible. If she “disappears” I’d pop round, if I couldn’t get in I’m afraid I’d consider making a call to social services, no one who has been involved in domestic abuse in the way you have witnessed wouldn’t take action given the red flags. I feel for you and your grandchildren and do genuinely fear for them and their mother. It’s a huge red flag if this new partner is already speaking to her in this way on your first and only meeting.

sorry to say but she sounds horrid saying that wow you have alot to deal with--I guess it is her life her child and she calls the shots anad as already said might be best to stay calm so thatyou can still see your GC and as importantly said keep an eye to make sure no abuse is taking place