What is a good way to deal with a friend who thinks they don't have any value?
Show them how much they matter. Always tell them your proud of them,Be a trustworthy person, listen to their problems etc
Just be there for them, and when they say something just turn away and be like "no, dude, if you're ____ then what am I? Gosh!" but say it in a humorous way so they don't feel bad.