Hi Cathy,
I am not usually definite in my statements .... most issues have shades of grey, and more than one side to the story ...... but on this occasion I definitely think you should just sit back and be a listening ear Mum, not a pro-active version! I don't think anyone will thank you for input at this stage.
You seem to be worrying about something which is not upsetting the people themselves. Your sons girlfriend is behaving impeccably, backing off rather than forcing a difficult situation, and respecting your grandsons' right to be with his father one-to-one. (would that all women were so tactful!!)
They need to work through this for themselves, and it may take a while. Let them do it in their own time. If someone asks you, then I think its fine to have an opinion, but until then - providing none of those in the relationship are distressed - I think all you would be likely to do is cause trouble.
If you simply cannot stay quiet ....speak with your son about your concerns, NOT his girlfriend.