There will be a few challenges of using a friend or relative to photograph your wedding. One is the fact that this person may spend, rightly so, a lot of time mingling with s/he's family and friends, missing important scenes taking place at your wedding. I believe it is best to have someone who is not related to you, a friend or any other guest relied on to create your wedding story with still photography.
I believe the equipment isn't an issue at all; why, most weddings I get hired, someone shows up with the newest camera, lens or gadget wanting to try it out.
Then what if the equipment fails, something as simple as the battery needs recharging, now what will you do? A professional has the necessary equipment, has operated it over the course of a long time, enabling s/he to be aware of the numerous emotional blips that occur at a wedding, anticipate the high, then photograph them.
A pro will also be able to do some directing to get the best light and composition, creating flattering and beautiful photographs.
Your choice.
What will you have/want to remember your wedding?