Perhaps some straight talking with your son and his girlfriend, separately or together, however you feel comfortable. Tell them both you are saddened by their split but their child is still your grandchild and as such you want to maintain and nurture your relationship with her and be very much part of her life.
If they are both decent human beings with an ounce of common sense and decency they will understand and actually welcome your involvement in the child's life surely.
What do you think would go well with coasters like this?
Baby Reindeer - anyone watched it?