A lovely sharp shot and great as a candid as he obviously has no idea you're taking a picture. But, from my point of view as someone who has recently got married I would have been really annoyed the best man was on his phone and not doing something a bit more useful !! So from that point of view this shot would have upset me a bit! I'm also not keen on the angle but thats just my preference, it looks untidy to me.
Maybe shots of the best man talking to the guests and generally doing best man "stuff" would be better.