I try to put it in the context of that person having worked there. I've often written something like this:
I recently spoke with Jane Smith, a co-worker of mine at Theatre X, and former Stage Manager at Theatre Y (where you're applying to) in regards to her time at Y. It was encouraging to hear about how wonderful her experiences while she was at Y, and makes me especially interested to speak with you about the possibility of future employment.
Something like that.
