J.H.
I thought the question was obvious...
Anywho, I'd just make sure you're smiling always and joking (until you get to know her). With my kids' friends, I started off joking and smiling and teasing them (and my own kids) whenever they were here. This served two purposes. The first was so that, heaven forbid, I had to get stern with them, they knew I meant business. Secondly, it put the kids at ease. Most of my kids' friends think I'm just a big kid with authority over them. Whenever one of my kids' friends need advice, and they're afraid to go to their mom and dad, they come to me.
Start off with that, and see if her thoughts change. If they don't, chances are she thinks most adults are "mean and scary" or your son is saying that so that you stay away from her.