Tenant/Sublease issues: What would you do?

[Shawn Q.]

So I have a strange one. I have a tenant that moved out - his lease ends 07/31. He's paying the rent and his uncle in town is looking for a person to sublease. They found a prospective tenant that falls under the credit cutoff (she's at a 512, my cutoff is 550). I will allow a co-signer to average up the score (in my area it's necessary as I have a lot of low or no credit renters - usually students) but her cosigner has apparently gotten cold feet. 

The uncle is now asking if he can be the co-signer. I'm leaning toward no for two reasons: I think it speaks to her risk profile if she can't find someone in her personal life to be a co-signer, and it would leave me at the end of the lease term with a tenant I wouldn't normally rent to (assuming she wants to renew). I have no written guideline for this situation, and the decision here would dictate what I write up, so I want to be sure to get it right. 

Any experienced landlords have an opinion.