Wedding conundrum

I would say you should do it... But you will need a few things

- Get a backup camera - a Canon DSLR that will take your lenses - borrow, rent

- Get a good zoom that has some range to it - 24-70 - rentals work as well

- Get a stand or two and another flash and some remote triggers - Cactus ones work fine

- Get several medium size flash cards 4-8 GB

- Shoot RAW and expect to post process

- Practice, practice

- Nail down where the ceremony is being held well in advance. What about inclement weather?

- Make sure you have an assistant that can get groups together and pose them, retrieve batteries, hold flashes, etc.

- Establish a list of shots you and the bride decide on ahead of time. Have them written down

I have shot a few weddings just as you described and they are a huge amount of work.  And even though you are doing it for free you still have a responsibility to do your best.  If at any time you decide you can't do it let the bride and groom know ahead of time so they can get someone else.