It depends on your and the couple's expectations. No expectations? No problems.

As the main shooter in getting high quality shots? Very difficult. Without experience? Pretty much impossible. Though it depends highly on the venue and how it is orchestrated. Outside before and after shots, pose shots, etc. are no problem, but the ceremony and reception are important. You need to get key moments where you are not in control, and having to run backwards/forwards to get the right framing will cost you (and will also be distracting). It's the same reason why wedding photographers use 2 bodies, so they won't waste time to get the right shot. Wide angles are great for telling the story, be it the Ceremony, the first dance, or subsequent festivities. Even if they are okay with portrait-only shots, you'll still be running around a lot or capturing them with deep DoF and cropping away resolution. In the least, it will be very stressful.

If I had one lens and they asked me to be the main photographer, I would decline and recommend some tips on finding a good budget photographer. I think it's a special day and would not want to capture something less than what I know I could do for someone I know. I've seen some horrible wedding pictures.

If you want to learn to do wedding photography, become a second shooter.