While there is no hard rule on this, I would offer a full refund, but that's just me.
While there is no warranty implied or otherwise, they shouldn't have fallen apart like that. He's had them for 5 weeks. total.
If you're building your business for long-term, I'd rather have someone like that telling his network that he found a really good seller on ebay, than telling everyone he knows what a POS he bought from some ebay Shyster.
Not your fault. No obligation to do this. I'm just saying I would do it. I've got a pair of magnanni oxfords I'm sitting on because there's one place where the welt appears to be separating. I'm afraid that if I sell it, 6 weeks later, someone will step out of the frickin' shoe. I pick it up and bend it every few days, but for some reason, it makes me nervous to sell it, even though I glued that small part down.