How much do under-run heels affect a horse?

My friend is interested in buying a gelding. He's a great horse in general, but he's got a few little problems, and one of them is under-run heels-- but neither she or I know how big of a problem that is. How do under-run heels affect performance? The gelding's toes actually aren't too long like I would have expecte, but the heels are bad-- the hair above his heels is almost touching the ground. Would they affect his ability to be ridden, and are they fixable?