The bride/groom can have as many photographers they wish at THEIR wedding paid or not paid.
If they are insistent upon having a friend be a photographer, that should definitely be discussed and agreed upon beforehand and stipulated in the contract. The bottom line is, the more of these photographers you have, the harder it's going to make the pro's job, because inevitably someone's going to interfere.
Out of consideration to the paid photographer I would contact that person about 1 week ahead of time and make them fully aware that you will be there shooting out of the request of the couple getting married. Ask if they would like to get together before the wedding and discuss some professional does and donts.
Again, unless the couple is insistent on this, I wouldn't really want to be bothered by something like this. I'm paid to get the shots. The friend? He/she is there as a guest. Big difference. I shouldn't have to discuss any does and donts with a GWC.
If you get snubbed by the paid pro then to hell with them and do what is asked of you by the couple getting married.
Not so cut and dry. Make sure you aren't violating the terms of the contract. You are really doing the couple a disservice by having a "hell with them" attitude. They paid lots of money to have a pro there to cover the day. The last thing you want to do is interfere with that.
It always amazes me that just because you are the "paid" photographer that does not give you 100% of the photographic rights to the wedding (unless you are demanding it of the couple).
Wait, so they are paying me thousands of dollars NOT to get exclusive coverage? As the paid pro, do I have a right to go all gung-ho and shoot down anyone with a DSLR? Of course not. Should I have exclusive rights to the money shots? Absolutely.
I do help out a professional from time to time and I am paid for it. That person lays out to me ahead of time exactly what they want of me. I also sign a waiver and must turn over all photos that I take to them in the end and must delete any copies I have once they tell me too.
There's a big difference between being there as a paid assistant/second shooter and a guest/friend with a camera, i.e. a GWC. Understanding that distinction will go a long way towards not being a nuisance at the wedding.