IIRC, most rides allow wheelchair bound quests to ride as long as they can transfer themselves from their chair to the ride vehicle unassisted. Although this very sad case has shown that there are possibly even more restrictions that may need to be taken into account for rides such as these. It seems that the guest not having muscular control of their lower legs may have been a factor in this and I don't recall rides having strict rules about something like that. They have rules about amputees and the location of the amputation point but that was all I believe.