I'm not trying to change the subject and sorry to the OP if I'm hijacking your thread.
A butt-plug is a good introduction to anal play, the other advice I would give her is use lots and LOTS of lube. I really can't stress that enough. I mean LOTS of it. Some guys put barely any on and think it's enough, it usually isn't. And the other thing she needs to do is relax, like completely, if she tenses up it can be painful or even cause an injury.
I'm definitely not an expert or anything, this is just from my own personal experience.