+13 sounds like an amazing handicap. The thing is though, he'd have no chance of winning any club competion at all playing off that. Given most stableford winning scores are 38 to 40 points, he'd have to go round in about 56 gross. In a medal a gross 57 might get him in the top ten. In matchplay I'd back myself to beat him since he'd have to give me about 18 shots.
Most plus handicap amateurs are close to having a good enough game to compete when they play well. One good round isn't enough though, you've got to do it on all sorts of courses in all sorts of weather, most weeks of the year. And you have to deal with the pressure of knowing if you don't qualify or miss the cut, you don't get paid. Two good rounds in an amateur event may win it.
There's a big difference.