If you haven't already said
no to the accepting payment; take it, it pays for your rental and effort. You can always make up for your poor preparation by offering a free portrait session with a custom framed enlargement (depends on how much they pay you). If you've already turn down payment, you can still offer a decent portrait shoot but unframed of course.
Don't let this one incident put you off shooting weddings, just be a little more prepared that's how photography works, with each shoot you become better ...just don't get down on yourself. If they are real friends i think they would understand BUT as long as you got those "key" shots, i would think your friendship would remain intact.
Personally i turn down every request from friends for wedding shoots because in my head i've already decided i can't shoot them and after i give them a few referrals, i tell them why i won't do their wedding
On the flipside when i get full pay for a wedding, i have no problems...go figure LOL (mind you i haven't done one since i switched to digital either {i've always picked up my MF for those}, thankfully as i'm still in the learning process of achieving "excellent" digital images, i spend most of my time working on my photoshop and similar skills, which to me is what i think is the most important part of the digital system.