There you go... good advice in big letters! From what I remember seeing of photos you've posted it seems like you're still learning; it might be better to decline and maybe offer some suggestions as to where the friend could find a photographer if you know of any.
Or, better yet... see if you couldn't come in as a second shooter for the hired gun!
There's nothing wrong with never having done a wedding. We all start somewhere, but John is 100% right.
Refer the wedding out, and then contact the photographer and see if they'll let you assist and shoot in-between helping schlep equipment, or see if you can be a non-paid third shooter.
My VERY first wedding, I came into Nashville to visit for a week, before we moved here, and my friend hooked me up with Myrick Cowart, who was super nice and awesome and let me tag along as a third shooter on a wedding, just to get some wedding experience.
He didn't need me there. He had a second shooter, but he was totally willing to let me hang out and shoot, and he had me assist him in between shooting, and taught me a lot. I gave him my images when I was done, but he was kind enough to let me use them for my second shooter portfolio if I wanted.
If shooting weddings is something you want to do, then you need to do is start networking with photographers who will let you assist, third shoot, or second shoot with them, before you take on one of your own.