One other factor to consider is whether the grandmother-to-be might think a text announcement is a bit impersonal. And that would be on the son as well as on the DIL. Probably more so on the son since that was the way he wanted to tell his mother.
If I were to receive a text that my son and DIL were expecting, I would find it a very impersonal way to share very special news with me. That would deflate my interest in the event.
Our son did call to tell us when he and our DIL were expecting. We had a nice, personal conversation in which we could share our excitement and tell him how much we loved him. His wife was not on that call. Her choice - of course - but she missed an opportunity to share wonderful news with us and create a bond. The kid is a year old now, and she has yet to mention the pregnancy, the baby, or the significant gift we sent. If I had only her to rely on, I would not yet even know I was a grandmother.
So maybe this grandmother's impersonal attitude is a reflection of the impersonal manner in which her son chose to tell her.
We had a death in the family not too long ago - elderly parent. I received many cards, letters and phone calls, and I deeply appreciated each and every one of them. What I did not appreciate, and was actually offended by, were emails from people expressing sympathy for the loss of my parent. How lame. My parent died, and they couldn't bother to pick up the phone and talk to me personally or send a card or hand-written note. That would be too much trouble. I'm sure it is somewhat of a generational thing with me, but to send a sympathy message electronically really rubbed me the wrong way. I am afraid I would have the same reaction to an electronic notification of my son having a child.