I think he should have chosen one or the other, and been happy with the choice.
Personally I would think a close family wedding trumps a game (why wasn't the son, presumably a cousin, also invited?) but if you choose the game then send regrets and a gift for the wedding and stay at the game.

He should have gone to the wedding. I assume they have assistant coaches to take over when the coach is ill or can't make the game in an emergency. He should have had the assistant coach the game. It is one day out of his many days coaching. It was only that day to see his niece get married. He didn't make the correct choice. NancyLouise

This is what happens when people try to have things both ways instead of making a choice. I agree that he should've chosen the wedding, but that isn't the point. It was trying to do both that made the trouble.

Some people haven't yet responded to my daughter's wedding invitation, and in a couple of cases I suspect it is because there are FOUR weddings in our community that evening and they are among the people that are invited to more than one. One couple, very good friends who are even closer with one of the other families, told us they aren't coming to our wedding but will come to one of the other weekend events. Another got lucky and didn't have to choose, because they have a family event out of town that day. Others chose our wedding. Our rabbi will have to leave after our ceremony to go perform one of the others -- they moved the time for him, so he has to! -- and then return for dinner. It will mean missing the appetizers, so his wife is still deciding what to do. :) But I know to order dinner for them.

But there is one couple that I have a hunch is going to try to do something complicated -- they often do. Honestly, although we would love to have them (I mean, we invited them), I wish they would just choose one and have done. I know it is awkward to have to choose (don't blame us -- we had the date first of all of them :) !), when it gets super complex, it starts to feel less like they are doing it because both weddings mean so much to them and more like they feel all their friends will be devastated if they don't choose them. I can live with a "no." I vastly prefer what my friend above did -- say no, and ask if there is some other point in the weekend at which they could celebrate with us.

What I really don't want is to get a "yes" from people, put them at a table, order food and liquor for them, and then find out that they aren't going to stay for dinner. That usually happens with someone who is sick or something at the last minute anyway, but please don't set me up for it just so you can play musical parties without deciding your schedule in advance.

What if there was a rule: before you can be a guest at a wedding, you have to plan and carry out a wedding. I think people would realize the messes they make with haphazard attendance.

Difficult call and I can relate to both sides.
First, as a coach it is his responsibility to both exhibit leadership and teach leadership. Half the battle of leadership is just getting people to show up; but the other half is learning to delegate and allowing others the opportunity to step forward and take charge. It appears he felt an obligation as a coach and he wasn't ready to step aside and turn the reigns over to someone else. So give him credit for trying.

But, his non-attendance at a close family members wedding gives the impression that it's not a close knit family or he doesn't count his family as a priority. Is he married--did his wife attend? Is he divorced- maybe weddings make him uncomfortable. Who knows what was going through his mind.

Nevertheless, arriving cross, eating and leaving is rude. That's a sad choice and a character flaw. For that there's no excuse. My thought is he should have gone to the wedding.

In two days our son gets married and I can attest that trying to estimate numbers for reception meals is challenging. There's a fair number who have failed to RSVP and a few that have said yes, but now cancelled and a few who have indicated they are bringing more than we invited.

.... and a few who have indicated they are bringing more than we invited? Was that ok with you or did you tell them unfortunately you didn't have the room?

When my daughter got married, we pretty much gently but firmly hounded anyone who did not RSVP. I figured if they did not have the manners to let us know one way or another, I'd suspend my manners too. We did have a major snowstorm that day, so the few out of towners who had accepted but did not attend, got a pass.

...and a few who have indicated they are bringing more than we invited.

Who are the extras they are bringing? If it's a case of someone in a relationship and your son did not invite the other half of the relationship, then that was rude, but still does not excuse the guests from bringing anyone uninvited. If the guests are bringing anyone else (a date but not a girlfriend/boyfriend, or children, or some random person) then they are way out of line. I do agree that many people don't realize that this is so rude until they've actually planned a wedding.