From what I know, if you have a well fitted one, they do help tremendously. I'll definitely wear something that'll keep my teeth in, that kind of dental trauma is like a personal nightmare for me...
Just so everybody knows, mouthguards' main use is to absorb the shock from your teeth smashing together upon crashing, landing, etc... reducing the likelihood of concussions. Although they are called mouthguards, they may not protect the teeth from something like what happened to OP's buddy, but it'd be better than wearing nothing, obviously.
he should get a couple fuckin saber tooth tiger fangs instead of real teeth when he gets them replaced. probably wouldn't be eating many girls out after that, but he'd look fuckin badass
Implants BLOW. They put me under for my 2 implants, and I woke up in the middle of it to a machine drilling in my bone. Thank god they numbed the piss out of me
mouth guards absolutely help in preventing concussions man don't know what "scientific research" you're looking at. But main purpose is of course to prevent the teeth from chipping and jaw jamming into your teeth, also protects your tongue.
Scientific research? You know, as in applying statistical methods to data? This has not shown a correlation between mouthguard usage and decreased concussion prevalence. This summarizes some of the current body of research: http://blog.medfriendly.com/2012/01/myth-that-mouthguards-prevent.html