The way I see it, you're a friend with no experience in weddings, so they just want "insurance" that they will get good photos. Are you getting paid for this gig?
During a wedding, there are many things that can get in the way. Caterers, videographers. In my last job, the videographers and I were vying for the best spot to get the bride coming down the runway. We talked about this before, and agreed to take turns. As long as you communicate, things should be fine.
Personally, I have no problem with another photographer coming on board, because I will take the reins as the "lead". I have the ideas, I do all the arranging of rings/shoot ideas, and I conduct myself in a professional and organized manner. I've been double-booked before, and it's the professionality, proactiveness, and endless ideas for what and how to shoot that distinguish yourself.
There will always be people in the way. There will always be wedding guests with smartphones or DSLRs standing up to take shots. It's your job to plan ahead and be ready for all this!