Rollergirl, just wondered who set the date for the christening - and at what points in time relatively did the christening date get set, your mum's operation, and the fact your friend wasn't going to be around.
I know it can be difficult to set a date that everyone can get to, recently my sister was trying to sort out a date for her dd's christening, having booked it a long time previously the vicar suddenly announced that he was off on holiday for a couple of weeks at that point and they would have to change it . Deciding the key people to have there were the vicar, themselves, god parents and grandparents, at one point it looked like it was going to push into late october because of various different commitments people had.
Luckily, they managed to find a date that only gave people short notice but most people were able to attend. I was one of the last to find out, when she rang to say the date was changing I said 'So long as it is any weekend apart from xxx' there was a silence at the other end of the phone as it was indeed that weekend. In the end we changed our plans (altho' dh was keen on sending me and ds2 off to the christening whilst he and ds1 did the stuff that we had already said we would do ) as she couldn't face waiting until the end of the autumn (not to mention they were having the party at home afterwards, easy in a big garden, not so easy to have a garden do in late oct!).
Guess this is a very long winded way of saying, who chose the date of the christening? If MIL chose it and went ahead without checking that everybody important to the ceremony could make it then she can't complain too much about people not coming.
You could always say to her that you don't think she is showing a very christian attitude if she is being unreasonable about things!