At our practice we (the midwives--not me, I'm the nurse!) have held up the lip, if mom has the urge to push, and if the lip is stretchy. Or we have guided mom's fingers to the lip and had her hold it up, if she can reach it.
It is not uncommon to have this problem, especially if there has been some sort of cervical trauma--surgery/biopsy, etc--and there are adhesions/scar tissue holding the cervix back. I don't think that someone is a medwife or is meddlling, to hold a cervical lip out of the way if there is an indication for it. Just mho, though.
Position change, GETTING IN THE WATER, relaxation and massage (of mom, not of the cervix or the perineum), food and drink, getting to the bathroom to void, encouragement, visualization, talking to the baby, mom feeling safe surrounded by supportive people, pushing how and when mom feels like it, all of these things are indicated for a persistent cervical lip that won't budge.
Any other ideas out there?
Oh, I don't think a mom should ever be left, extenuating circumstances aside. If you need to take a break, someone else needs to be there for mom, even if they are just sitting in the corner out of the way. Most doulas I know work in pairs, so that if it becomes too intense, or too long, they can trade off and someone can get a break. Unless, of course, mom tells everyone to get the hell out; then it is okay to leave!