Sorry, I'm probably not going to help but I think it all depends on what your friend is in to. Some people like fast straights, some like slow(ish) corners or somewhere were they can see for miles.

I'm in the same position (wanted to marshal with a newbie) so I sat through a onboard lap with my mate and got him to say what he liked. In his case he liked technical bits so I'm thinking bradden bridge/oak, union mills or bungalow (as I've done them before).

Some places might be a bit 'intense' if he has never marshalled anywhere before...

Don't worry too much though - I have found something good about everywhere I've been so he'll have a great time!