My cousin asked me the same thing back in 2019 when he wanted to go to law school and I had just started making okay money at my job. I told him no and it was awkward for like a year, but then he got a job at a firm and paid his own way part time and honestly he was better off for it. Co-signing is not just a favor, it's your credit and your money on the line if he stops paying, and grad school loans can be huge. Your old roommate is still just a roommate from your past, not family, so you don't owe him this. If you can't say a straight no, maybe offer to help him look at other loan options or schools with better aid instead.