There are all some very good points here, but I think
@Box Brownie has a great one - are you willing to risk the friendship 'going south' if things go wrong even if you are not to blame?
I actually shot weddings for a studio many years ago, and it can be very stressful. Whether it's trying to round up the wedding party after the ceremony that just wants to go barhopping (usually it was with the couple that didn't want to see each other before the ceremony...that just happened to live together...
), trying to be in multiple places at the same time (believe it or not, we didn't use a second shooter!), hoping you got your exposures right (yes, film!), or any other myriad of possibilities that could go wrong. I do know the owner of the studio had to pay for basically a complete redo (hair, makeup, tuxes, flowers, venue...you get the picture, pun fully intended) when the film from another shooter was unusable. He didn't disclose if he had insurance to cover such a possibility, one would hope so. Honestly, I don't think I would ever want to photograph another wedding!
Sorry for the long read, but I do have one mildly amusing story. For reference, we were required to dress up, men in tuxes, women in nice dresses or pants suits. Also, we carried two Hasselblad 220 film cameras, and a 35mm for "insurance" shots, along with strobes (Vivitar 283s, if memory serves), film, batteries, etc. To carry all that, I bought a black roller bag and cut out a foam insert. I usually set that on my passenger seat with my primary camera on top, ready to go. I had a wedding on a military base right after 9/11. The base had staggered concrete barriers, so you couldn't drive straight through, you had to weave between them. In between these barriers were checkpoints - first one, pop your trunk and hood. Second, mirrors examining the undercarriage of your car. Can't remember the third, but the officer at that station ducked his head down to my driver-side window and told me I was free to go on. I looked right at him and said, "Funny no one thought to check this big black bag right here." The look on his face was priceless!