Perhaps a (difficult) talk is in order between him and her. She may have expectations that will get disappointed later on down the road. Better to get things on the table now, before she gets too invested in this. Something to the effect that his involvement in terms of time and emotion will be limited. She should decide if that's something she can live with.
If you guys keep the communication channels open, I'm sure you'll come out okay. There may be some more bumps in the road first, though.
Love means never having to say, "Put down that meat cleaver!"