I have found that being a guest, and a worker (even un-paid) at a wedding is very awkward at best, and puts you in a situation of losing a friendship at worst (if critical pictures don't work out).
It doesn't matter much that it is supposed to be a low-key affair.
That said, years ago, I vowed I'd never do it again, but guess what? I'll be doing a friends wedding next month. Do consider tellng your friend (nicely) to hire a pro, if you have any doubts.
But if you decide to press on, you have a good selection of gear, pick what you are most comfortable working with. Go to the rehersal, especially if you aren't familiar with the space.
Talk with whomever is officiating and find out how he/she feels about photos during the cermony, etc. Figure out where your best vantage points will be, so you aren't discovering everything fresh the day of the ceremony.