what an interesting opportunity! i've been thrown into similar situations many times. your friend obviously has a lot of confidence in you. i never tire of this kind of stuff. i walk in giggling. hopefully it happens again some time.
The same thing happened to me, and I didn't know any of the guys. i found them on Craigslist, and when I spoke with the guy he said he didn't want me to hear any of the tunes so when I got there it was more "spontaneous". It worked out well, and I actually was impressed with how it came out the first time around. We recorded it right then and there and I had a CD to listen to on the way home. Kinda neat hearing it so quickly.
I guess it depends on the music. We played straight forward rock and roll in 4/4 time. Any drum part would've fit. I played along with fills here and there. There were some issues with bridges, but all in all it worked. But, honestly it's a bit insulting to drums as an instrument to say, "just show up with your kit and play'. Makes it seem like it doesn't really matter all that much. How did it go?