Accidents happen, and I don't think R's disability is really relevant here.

If you think about it, riding in a group is a highly complex social effort. If R's dad (who you say is very experienced) didn't feel confident that R could handle the social and communication aspect of the group ride (or this particular group ride), they would probably choose to spend their time and effort doing some other activity. Or they would be on a tandem with R as stoker.

If R's skills were such that he might be jeopardizing the safety of others in the group, I hope someone would have said something before something happened . . . but from your description it sounds like that wasn't the case.

Hopefully T doesn't make it an issue.