As far as I know, if he's bowling right handed, nothing changes. His average is his average. One step or 8 steps makes no difference.
I think he's stupid for daring to bowl injured. He's still sliding on his injured ankle.
(Is this a macho thing for him?)
There'll be a little less force due to less steps but it's probably on the order of 75%. He's more likely to damage it worse than he is to help your team. It will also likely delay the healing process; so it will be longer before he can bowl properly. I'd get a sub or convince him to allow you/the team to get a sub for him until it heals properly.