I can't tell you how many weddings I have attended as solely an MC because the couple hired a DJ who was their friend and only charged them $200 to do their wedding.
I have to give them credit for hiring an MC, though, because at least the MC can keep things on schedule.
However, the DJ usually had a very unprofessional setup with drinks right next to his DJ controller, as well as cables that were laying on the ground freely.
The only time it would be acceptable to hire a friend is if they actually provide their service for weddings regularly. I have DJ'd many weddings for friends and we always treat them the same way as a regular client, and it's always a lot of fun!
We won't be more lenient with the service that we provide because that's not fair to them, and we want to maintain consistency across every wedding that we do.