It sounds like your friend's one comment is being blown way out of proportion IMO. Like you said, just because CC is something that worked for her boss and her colleagues doesn't mean it's right for everyone. Your friend shouldn't be made to feel like she is because of her parenting choices. Everyone is entitled to personal/sick leave whether it is to look after a child or because they are sick themselves. I presume she wouldn't be made to feel like this if she had been ill herself and had time off? Or maybe she would! Whether or not she has the right to return to work part-time after parental leave seems irrelevant as this is something her & employer have already agreed upon ITMS. From your post it doesn't sounds like your friend's boss has a problem with her work or even her work hours - more the fact that she is 'going on about mothers' rights'. It seems like your friend is really wanting to know if part-time workers are entitled to the same/equivalent benefits as full-time workers ie payrise.

The text below is from the direct.gov.uk website about part-time worker protection which has got heaps of information about returning to work after parental leave and flexible working options.

You should receive the same treatment at work as an equivalent full-timer, so any job benefits, employment terms and conditions or opportunities open to full-timers should also be available to you. The benefits are normally ‘pro rata’, meaning that they should be in proportion to your hours. For example if an equivalent full-timer gets a £1,000 bonus and you work half the number of hours, you should get a £500 bonus.It may not be possible to pro-rata some benefits to part-timers (for example, health-club membership). In this situation your employer would have to decide either to give the benefit to both full- and part-time staff or (if there was objective justification) not to give part-time workers the benefit.
Your employer is entitled to offer better terms to part-timers, perhaps to encourage a more balanced workforce, but they will need to be sure that doing this is not against other discrimination laws.
Parental leave and flexible working : Directgov - Parents
Part-time worker protection : Directgov - Employment
Hope those links are ok as they're .gov ones?

It's a tough situation as I'm sure your friend doesn't really want to confront them as this will probably only give her boss more 'reasons' to try to force her out. So I haven't really got any advice for your friend other than either to decide whether to carry on, keep quiet and try to work through it or decide to pursue some sort of discrimination action against employer if that's even possible. Or look for another job....