Being thrifty is a good lesson to learn at a young age. We always tried to instill that value in our boys. I’d say it worked. My older son makes about 150K per year yet he still has the same car he bought after college 10 years ago. He’s says it is low mileage and works fine. Same with the other son. We go to thrift stores frequently.
Being poor is something you never forget.
I think she was trying to teach him humility by going to Goodwill. I would also make him volunteer at a food bank or serve dinner on Thanksgiving at a church. A 13 year old who makes fun of poor people is heading in the wrong direction.