I have a mutual friend with a job candidate whom I've never met. Is is appropriate/legal to ask my friend about the candidate?

Title pretty much says it. A friend of mine went to the same college as a job candidate that I'll be interviewing in the upcoming weeks. Is it legal to ask my friend about this person if they aren't listed as a reference? I read a post in r/humanresources where an interviewer had the candidates sign an agreement that they could contact whoever they wanted, which made me more unsure than I already was. If it is legal, is it appropriate or recommended? I am in Pennsylvania.