I totally understand those concerns Fimac, of course, but it really is not anyone's place to try and change a woman's mind about the way she conducts her pregnancy unless that woman is completely ignorant of risk factors/potential consequences etc and walking blindly into a situation.
From what pruni has said this would not appear to be the case here. Her friend has hired an independent mw and I would imagine has an honest and open relationship with her (that is usually the case with the one-to-one care an IM can offer). And the midwife would not be doing her job properly if she did not keep pruni's friend well appraised of all the possible consequences. She would be looking at the situation as a whole, not just focusing on what the calendar says. If it has been a healthy pregnancy so far and the mother is happy to 'go to 43 weeks and then think about it' I would see no reason why she shouldn't be supported in that decision.
I don't have the exact stats but I believe that something like only 5% of babies arrive on their due date. That's an awful lot of leeway either way. My cycle certainly isn't a cut-and-dried 28 days, ds was delivered at 42 weeks, there was no issue. Dates are often incorrect, and as others have said other countries don't even use the 40-week marker at all.
Of course there are 'what ifs'. And I'm sure pruni's friend has thought about them all. Unfortunately, nothing is for sure in this world and when a subject like this comes up, there are always horror stories to tell. There are also plenty of happy endings too.
Hope your friend has the labour and birth she wants, whenever it finally happens, pruni