Only teach when she asks for advice or seems very frustrated with her throw. Even when she is obviously doing something that is killing her distance or is technically against the rules(ie stepping in front of her marker). If she is having fun, keep your mouth shut and let her play. And the hanicap thing is a great idea but even that can be thrown out. Just let her compete against herself if she wants. My wife knows I'm more consistent,(I practice she doesn't) if she was trying to beat me even with a handicap I think it would frustrate her. Having a disc that she can be consistently STRAIGHT and hopefully FAR with is also very big. If she's always in the trees she is going to hate it.
This is good advice, my gf had a lot more fun at first when I only gave her advice if she asked for it. Now we both help each other with form, but we still try to keep it from being a constant thing.
Mulligans are a great idea volklgirl! My gf takes a mulligan for every 9 holes we play (more if she's having a rough day), and it really helps her enjoy tougher courses more if she doesn't always have to play from a terrible lie.