Like coachmolly said, sometimes just thinking about stretching up tall (i.e. shoulders pushed up by ears, core tight, standing as if there is a rod going from the bottom of her foot to the top of her head) before beginning the BWO is extremely helpful. If she stretches tall through her body, she will have less room to veer off to one side, and she'll be focused on her body alignment. Also, it can never hurt to take it back to a line on the floor or to low beam and just repeat, repeat, repeat to engrain the motion into her muscle memory.
I had difficulties with BWO on beam as well when I was competing level 6. A lot of it is because I would do exactly what your dd is doing, going off to one side and being unable to get the first foot on. I also had a little bit of a fear of the skill. My back flexibility wasn't great. It was just one of those skills that didn't ever really get along with me. I did get it down eventually, though, and luckily I never had to compete it again. It's one of those skills that you can leave behind if you have to. It's helpful to have a good BWO for the level 7 series, but after that you don't need it anymore. Not that I'm suggesting that she should give up on it and decide she never wants to compete it again after level 6, but it's nice to know that it's not required in future levels.
Good luck to her! This skill in particular takes a lot of time for a lot of gymnasts, but I'm positive her perseverance will pay off.