Oh, man. I have this problem all the time.
I've been playing long enough that it is very common for me to play someone I know. I guess after a while you get used to it, but I can understand why you are nervous. It's easy to get sucked into overhitting or getting cute or anticipating what the opponent will do based on what she has done in the past.
The classic advice is this: Play the ball, not the opponent. Don't overthink it. See ball, hit ball. If you start missing, focus even more on the ball.
Avoid chitchat before and during the match. No need to be rude, but focus on what you are doing well and what needs to be done better.
And remember -- she may be equally nervous playing you!
-- Random Error Generator, Version 4.0
-- Master Moonballer