I agree with those who say no. Agree that you should kick in what you feel comfortable contributing, if that’s something you can do. But don’t co-sign a loan.
I have had former students default on co-signed Grad PLUS loans. Because the credit requirements for the parent and graduate PLUS loans are very loose (you don’t need good credit -it just can’t be bad), it’s bad news when credit is declined for these loans. What I found was that the students who had to get co-signers were pretty likely to get behind on repayment. It makes sense - unless they learn new patterns when it comes to budgeting, they are likely to fall back into bad habits. While this is not true for everyone, I saw enough of it that I would not want my credit tied to theirs.