I've spent some time thinking about this one, as someone who is gratefully able-bodied but worked for many years with people with various disabilities.

I think if R has been riding with this group for some time, if they are experienced riders, and the rest of the group does not see his disability as something important to tell a new rider in the group, then it isn't.

If T believes otherwise, then it's only prejudice speaking.

I have to comment though: if everyone in the group was very familiar with the route and knew there was a narrow bridge upcoming, then "CAR UP" might've been enough. But normally, "CAR UP" means "stay on your side of the yellow line and be wary about the possibility of the car weaving into your lane." It does NOT mean "braking, stopping or slowing." If no one said or signalled anything other than "CAR UP" and everyone but T, the new rider, knew that that meant the group was stopping, then that might be your problem right there.