Gelish can be bought in Sally's, all you need is a manicure certificate. You would need to know that the person offering these treatments is not in possession of this, but tbh, I don't think anyone would bat an eyelid about it. Unfortunately.
I have had my nails ruined by a qualified beautician who previously was a spa salon manager too. I'm 100% sure she was qualified, but she still damaged my nails badly.