Hi Jill,
I used to have an AMAZING volunteer with CP who was in a power chair. She did data entry for me and while it took her a long time, she really enjoyed the work and was one of my constants. (She also got herself a master's in accounting in her free time which tells you everything you need to know about her drive and motivation!)
We've also had a volunteer with similar communication challenges serve with us a greeter in one of our gyms. His speech was a little slurred - he didn't use a communication device - but offering general greetings to everyone who came in was something he enjoyed. And most importantly, he felt that his presence contributed to the guest experience.
Something you've likely already thought about: if you have her in a space like an information desk, be sure she's not partnered with someone who will always step in and respond to people first. She needs the opportunity to be approached and respond, and maybe there's someone else nearby in case the need for response is urgent or for some reason she needs the backup. I'm sure you'll find a role for her where she can shine and develop those skills. Good luck!