Wow. Will your mower even touch that now? You say he got hurt in a "batwing incident"? How hurt, and is his mower now down? If I thought I had a mower that could handle it, I think I'd just go mow it and not even mention it to him. He's your friend, and now sounds like he's hurt. Sounds like "favor time". If your mower can't handle it, how much can you rent a flail mower for in your area? Don't know how much ground you need to mow, but maybe a weekend flail rental can take it all down?
If he's already hurt, and his son is trying to mow now, sounds like a recipe for another injury. If it was my friend/neighbor (mine is both to me too), I'd just mow it already, and act surprised when he finds out, telling him "I though you mowed it?"