This is tough because it is someone else's daughter, not yours, so you have to be very careful.
My 3 year old has a speech delay. I said something to my ped when he was 18 months old. She did what she could to rule out other issues. She had his hearing checked as well, just to be sure. She told me that there wouldn't be much a speech pathologist could do but that she would refer him if I wanted to (and that she would have her own child evaluated if it were here, just to be safe). I did have him evaluated. He was evaluated at 18 months, again at 24 months and again at 2 1/2 years. That's when he started therapy (not much they can do with him before then except give me ideas).
I realize that what many other moms said is true - there's probably nothing to worry about, she probably just has older siblings talking for her, she will probably grow out of it and start talking soon, lots of kids have a sudden "language explosion" and never stop talking.
I knew the chances were my son would be just fine without any help. I was not willing to take that chance. He has been receiving services for about 9 months. He is doing much better, saying more things, speaking more clearly, more and more willing to try and say things. He has a ways to go, and it's obviously that he gets really frustrated sometimes when people have trouble understanding him. But over all, it was absolutely the right decision.
What can you do? Probably not much. The first thing that comes to mind is just that if you are there when she is obviously trying to communicate and is frustrated, you could make a very understanding comment either to the girl (that Mom overhears) - "It's hard when people don't know what you want. Can you try telling me again? I really want to help." - or to Mom - "Oh, it is so hard as a mom when you know she wants to tell you something and she just can't communicate!"
I realize the hitting and kicking is also part of being 2 1/2, but that doesn't mean that she isn't frustrated. It must be so hard to be that age and not able to communicate your needs.