Then do it. Worst case is she gets laughed at when she goes back to workBoss is very cool. No risk there.
Honestly, if it's a laid back-play what/when you want type gig then I think you will have fun. I'm sure most people will be light hearted IF you made mistakes, knowing you're a fill in. That is, if they EVEN NOTICE.
If you are THAT uncomfortable about it, then no sweat respectfully declining. If you do it, we want updates