My grandma still mails me a birthday check every year and I hold onto it for weeks because I keep forgetting to go to the bank. Last one sat on my dash so long the ink faded.

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.