It all depends on how much your friend wants to spend and how far they want to travel from Edinburgh or Glasgow (the two main airports).  Carnoustie and St. Andrews along with Glen Eagles and Turnberry are about the most famous, but there are incredible courses all over Scotland.  Golf doesn't have the snob appeal there that exists in the States and virtually every small village has a course that's good compared to courses here.  

 My suggestion would be to research one of the big courses, like above, and go for a stay there or if your friend is comfortable moving around doing one of the golf tours.

You can't lose either way.