I can't give you it from another mum's perspective, but I can give you it from a viewpoint probably quite similar to your son's gf.
Boyfriend and I have been together for 3 years, I'm now 20 weeks pregnant. When we told his family, they were fine with it - they said as long as we were happy, they were happy. Boyfriend was happy, I wasn't so sure - thought everything was over as I'm at uni, got a good social life, great friends etc.
Told my parents and they told me I should have an abortion. At that point I wanted to leave - we went to the student house we share with our friends and stayed there. I didn't want to speak to either set of parents - even though my boyfriend's family had been supportive, I wanted to be far away from anything that reminded me I was pregnant while I tried to deal with it, and I pushed my boyfriend away too. I knew how much he wanted to keep it, and although he promised me he'd stick by me no matter what, I couldn't quite believe that it was true. I felt like I was trapped and I didn't want to speak to anyone about it.
People from both sides kept trying to get in touch with me, but save for letting them know essentially that I was still alive, I didn't want to talk to anyone from back home. That may be why the gf was so reluctant to talk to any of you about it, including your son - I didn't want to talk to my boyfriend, because I was sure he'd try and convince me to keep it, and I didn't want to be swayed - I wanted to have time to come to my own opinion.
Eventually, everything has settled down. We've both spoken to my parents, and everything is really back to how it used to be - except that we're closer now, because they consider my boyfriend family too, whereas they never used to. They can't wait to become grandparents and keep telling everyone how excited they are - and I know now that their reaction was one of shock, rather than anger - they'd always assumed that if I got pregnant young, I'd have an abortion. I'd assumed that too, but it isn't that simple when you realise that it's going to be your child - suddenly I realised that I'd probably never overcome the guilt if I had an abortion, and that I'd (wrongly) blame my parents - and I didn't want our relationship to be destroyed forever.
Things are going to be tough for us, and my boyfriend and I have had to grow up so quickly, and the toughest times are still ahead - I'm well aware of that - but I think the best thing we have right now is an amazing support network. There's no way we could do this without the love and support of our families - it's been an absolute godsend to us.
I know this has turned into an essay but I hope this provides some kind of comfort in that it may be an insight into the mindset of your son's gf, and maybe a little bit of proof that you're not alone.