Some people finding walking meetings to be really beneficial in terms of boosting their thinking/focus while talking. Maybe it even makes her a better therapist. Who knows?

I'm one of those people that tend to pace when on the phone and sometimes use a walking pad treadmill during online meetings. Here's an interesting article about it: Stanford study finds walking improves creativity | Stanford Report

But if it bothers you, the client, then definitely say something. It's your session and she should only walk during your session if you're okay with it.