Like Tea1Sugar, I became pregnant at 21 when in my 3rd year of university, and had by baby aged 22. My ex asked me to have an abortion (although has subsequently denied this) and we split up when it became clear we had different ideas about the pregnancy. I didn't hear from him again for years, which is the big difference between him and your son. With hindsight, the shock of an unplanned pregnancy can cause such a kneejerk reaction just because it's an option, not necessarily because it's what either parent wants.
My ex then turned up a couple of years ago wanting to see my DC. I have so far refused, they're in their GCSE year and it could cause damage (although they have always known the basic facts). Not to mention that I have my younger child (13) to consider, and my husband who has raised my elder as his own since they were tiny. Not to mention my ex's other children (14, 12 and 4), who haven't a clue. Not to mention me, or my ex's new wife! There are lots of people to consider when a parent leaves, especially when they walk back into their child's life many years later (which they invariably do!).
Well, my little rant at my ex is over, and thank you for allowing me to hijack a small portion of this thread. ;) I think my point is that he needs to make sure he is there doing all he can, supporting her and doing everything to be a father to his child. I know she's not making that easy, but she is hurt and no doubt feeling abandoned. He needs to keep trying to show that he means it.
If he doesn't mean it, and is just behaving as he thinks everyone expects him to, then TBH he would be much better staying away. A half-hearted father can cause much more damage than an absent one. It sounds as though your son is a sensible chap and I am sure he will stand by her.
And as for you, you need to make sure your son's ex knows that you will be there to support her no matter what. Your grandchild needs you more than your son does at the moment.
Echoing what Tea1sugar said, their lives are far from over, they just need to find a different way of achieving their objectives. It might take longer, it might be harder, but it will be worth it for the end result.
A grandchild is such a blessing, once things have settled you will forget all of this angst. Many congratulations, and (slightly patronising) well done for caring and trying to do the right thing. :)