I was recently talking to a couple who is getting married and when I asked them who was going to photograph their wedding they said "a friend who is a photographer." Not as a paid professional photographer but as a friend. This isn't the first time I've heard this from a couple and at first brush it sounds like a great idea, mostly because you'll save tons of money, right? But I'm going to ask you to consider these things before asking a friend, especially a friend who is a professional wedding photographer, to take the photos at your wedding.
"what end result do we want? what kind of photos do we want?"
A paid professional wedding photographer will make sure that there are images of all the important elements of your wedding. If you don't really need photos of every detail and all the moments then you may not need a professional photographer. However, if your answer to this question is "we want all the details and moments captured!" then expecting your friend, who will also be a guest at your wedding, who may want to catch all the details and elements but they may also want to have a cocktail and talk to their friends who are likely to be at the wedding, might not be the right option for you.
"how would we feel if we were that friend?"
This is a big one. Depending on how solid your friendship is, that friend may not want to disappoint you. They may even genuinely want to do it even though they know they'll be distracted and are likely to have to choose between being a guest at the party or being the photographer. But it's a lot to ask a friend, especially one who does this for a living, and if you're expecting that they'll give you a big discount or even do it for free, woah-nelly! It takes a lot of work and time (not just the day-of but editing the photos afterwards), and it's a huge favor to ask a professional. Some wedding photography costs around $5,000 and if you're expecting that for free, that's a heck of a lot to ask a friend!
"do i want my friend to feel like a guest or a vendor?"
Know that the friend you're asking really can't be both. Consider this, a paid, professional wedding photographer is invisible (and sometimes comes with a second-shooter to make sure they don't miss anything). They blend in and go pretty much unnoticed. They don't chit-chat, they don't drink. They run around, they're sometimes lying on the ground or far away from the crowd to get the best shots. They're always on the lookout for that next best shot. They (sometimes) don't even sit down to eat. Now consider a guest: they enjoy mingling and talking to old friends, they enjoy a glass or two of bubbly, they want to spend time with you and celebrate with you, they probably want to enjoy the dinner they've been invited to eat. Well, you get the idea. The two cannot be one, it's either one or the other.
"would it make sense to hire our friend?"
This could make the most sense. if you have a friend who is a professional wedding photographer* it might be best to hire them just as you would any other vendor. Sign their contract, talk specifics, share your expectations, in other words: treat them as you would any other vendor, just end your meeting with a hug instead of a handshake (they are friends after all!).
Now, don't get me wrong, some awesome, amazing professional photographer friends can do it all. I just want to offer some food for thought, in case you hadn't thought about it this way.
*just a caveat that not all professional photographers shoot weddings. You should ask to see photos from some of the weddings they shot to make sure you'll like their wedding photography style.