I have never shot a co-workers or close friends wedding. I have shot numerous acquaintances though (my friend knew a friends sisters brother that needed a photographer type deal)
The only problem I have ever had was at a wedding I was shooting for a girl I used to go to high school with (didn't even know we went to high school together) apparently she had a friend who was a 'professional' photographer taking shots as well. I didn't really mind, I just kept doing my thing until after the ceremony we were grabbing some quick shots at the alter with just the bride and groom. I had about 15 mins to clear the room for the hotel workers to come in and clear it. The 'professional' photographer friend started directing and posing them

I kept my cool though, took a few shots and after we blew our time limit out of the water I explained we needed to move on as we are on a strict deadline
the other 'professional' photographer was pretty pissed but bride was happy and cool so whatever
I was pulling my hair out after that though