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.
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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