How should I approach a cosigner?

[Marvin Meng]

Hi, I realize that this is legal advice and that I'm in Illinois, which is a legal minefield. But I'm figuring probably some of you have been through this before and I'd like to know what your experiences were. I have a tenant who is habitually late (but eventually has always paid). He has not paid the September rent yet but says he mailed it (he always says that). 

Since there is a cosigner should I be calling his cosigner, sending her a more official notice, or serving them both the "Five day notice to quit"? I think once I rattle the cosigner's cage she'll do the rest but I want to keep my ducks in a row in case this does wind up in front of a judge.

Thanks!