Renting to couple not meeting income criteria

[John D.]

I'm renting a property for $1600. A couple wants to rent; but the woman has income of $19k while the man is attending college full time to get Physical therapy certification so no income. They have 10k savings. Mother of the guy is willing to be a co-signer to meet the income criteria.

What are the risks of renting to the couple with a co-signer?
What documents does the co-signer need to sign?
Who is responsible if rent is not paid?
Any differences in eviction procedures to be followed with a co-signer? 
Anything else I need to be careful about?

Thank you 

John