I think it needs to be highlighted. It was an empty wheelchair, the chair's user was not in it at the time. (edited for wording)
I do think it was shitty, and I think maybe a suspension from the team would be fine, but I also think we should show some restraint. They were at a bar/club/party and very possibly inebriated (not that that makes shitty behavior ok, but I mean look at the Hangover movies or the worship for old rock stars and their awful behavior, we absolutely, as a society celebrate drunken assholery), and they basically tapped it/pushed it a bit and it rolled and then flipped over going down the stairs. They didn't jump in it Jackass style or set out to see how much they can destroy it (I'm guessing they didn't realize how much it could damage it). He hasn't tried to excuse it (even by saying "it was dumb and I was drunk" or anything), even if he did seem to just offer a legalize "I'm sorry for my actions and those affected", but we've definitely seen a lot worse responses.
There does seem to be some other possible behavior (since he was kicked off of Arizona State team for behavior), so some scrutiny might be warranted. But, as shitty as it was, I don't see anything there worthy of making him a total pariah over.
Make him pay for the repairs/new chair and some community service with differently abled (edited for phrasing) persons, maybe him and his dad use their status to do PSAs to bring awareness to how much simple stupidity can impact other people.
Maybe, if they (by they I mean the person who's wheelchair he damaged) are willing, have him spend some time (like a full day or more) with the person he affected by damaging their wheelchair so he can understand how such behavior can impact that person.
If I were the kid's dad I'd like to think I'd offer to take the victim and her family on a nice outing (whatever her interest is, amusement park, concert, sports, or whatever) and then make my son have to see the difficulties they have to deal with in the process so that they can understand why pushing their wheelchair down the stairs is so shitty. Hell, maybe buy her a new wheelchair and make him have to try to get around using the old one so he can see first hand the struggle.
I'm glad some people mentioned the fucked up situation that necessitated the (edited for phrasing) person leaving their chair at the top of the stairs in the first place. Weird the bar singling out the behavior, they should face some scrutiny for their facilities. I'm guessing there might have been a police report or something and so the bar did it to try and save their own butts?