It sounds like you two were close and that you really regret that you have drifted apart. I think that it is definitely worth trying to rebuild a bridge between the two of you so that you can reconnect and try to regain what you had in the past. Maybe let her know that you really miss her, that you understand that it's difficult for her to be around children, and that you are willing to catch up with her without your kids. Possibly suggest catching up for evening drinks or dessert after your kids would be in bed with your DH watching them. Plus, it might take off the pressure for her because that's more "adult time" than "kiddie time". Let her know that you'll avoid talking about the kidlets, but that you are there for her if she decides she would like to discuss her fertility problems with you.

You might find that she wants to reconnect with you just as much as you with her, but that she's not sure how to do it after distancing herself from people.

I recently reconnected with a friend from Uni that I hadn't spoken to in about 3-4 years (although our problem was just laziness on both parts with keeping in touch). The great thing was that when we caught up we fell straight back into old habits and it was like those years hadn't even passed

Good luck!