Rosemary,
As someone who has dealt with infertility twice and is now expecting my second, I will say that this can be a really sensitive topic made worse by well-meaning questions/preoccupations from hopeful grandmas-to-be. You think she has been trying? Or you know she has? She's being referred to a specialist for testing on what exactly? Was this information she openly shared with you without prompting? I know a mother never stops worrying, but there is literally nothing you can do for her in this process, save for listening (if she comes to you and wants to share), or offering financial assistance for treatments (if that's something you're in the position to do).
My MIL constantly asked why I wasn't pregnant a year into marriage and constantly reminded me that she was ready for grandchildren. Then, she started to make me really uncomfortable with probing questions and suggestions of specialists to see, knowing nothing about the situation or what kind of medical interventions we were already investigating. I considered this to be in the same category as asking someone about their sex life: crass, inappropriate and out of line. Once I did get pregnant, she had a very difficult time respecting boundaries and we ended up sharing less information than we otherwise would've.
Now that I'm finally pregnant again, my husband I and are hesitant to share the news until we have no choice (when I start to show).
This is a safe place to share your feelings and worries, as long as those feelings and worries are staying here. I would just keep in mind that they have a right to privacy, probing questions only add to infertility stress, and as much as you want to help, this is not an area where you can actually do anything. Trust that your daughter is an adult and leave this to her.