Rat problem

[Syco]

My daughter has a male Siamese rat as a pet and was wanting to breed him so that she would have one of his babies when he dies. We got him a female and she has been with him for 3 weeks now and is starting to get plump, I'm expecting her to have the babies in about 2 weeks. The problem is... He won't leave her alone, he is still trying to mate with her and according to my daughter is very aggressive about it.They get along really well and cuddle together during the day, but she says as soon as the lights go out at night he is on her and won't leave her alone the rest of the night. Will this cause any harm to the female or the babies? Should I seperate them? I hate to because because he was an only rat for almost a year and he really seems to enjoy her company, well.... I guess that is obvious I've never noticed this behavior in my mice, once the female is pregnant they seem to leave them alone, but then again they have other females to bother. Maybe I should add another female rat to keep him busy. What do you guys think?