OP should not do her mum's friends' hair for free. It takes hours, it was dropped on her, she was giving very generous mate's rates, she was being messed around with the times and there's never a similar favour back. I'd have expected at least babysitting for free also a couple of days a year for the whole evening then, so I could have a date night with my DH. But why should OP do all this work for free? That's taking the Mickey and not appreciating the work that goes into hairdressing. If OP were a chef, would she be expected to cook a "Masterchef" dinner at the drop of a hat for mum and friends whenever they decide they want a special night out? What if she were a tailor? Should she do all this for free not only for her mum but also her friends? The acceptable thing to do, in my opinion, is to always pay something. If you are given discounted rates that's great and you should be grateful. I think your mum should have brought you something at least every now and then as payment for all those free hair-dos. Obviously, your mum was making herself look good by saying she arranged things and maybe even hoped the friends would be paying for her night out in exchange, absolute freebie.
Nobody has mentioned and maybe you don't care but I would be fuming if somebody dropped 4h work on me whilst on maternity and then not see I would be doing them a favour. I was knackered all the time, I tried to sleep when my baby did, I was constantly feeding, changing nappies, cleaning the mess after feeding... If I had to fit 4h of non stop almost-free work I'd expect them to be grateful, to help with baby, to make her own drinks and definitely bring something like wine, chocolates or a baby present. Your mum knew she was taking the Mickey, she said it was such at great bargain and also upgraded the hairdressing to pampering, as she were expecting also nails or something else. £30 for 5 people is cheeky when you were asking for £40-£60 to make it worth. Messing you with the time is not right either, you might have a schedule with baby and I assume that they arrive at 4pm, they would not leave before 8-9pm, which means OP would have to make arrangements for dinner, bath, nigh time routine... To be fair, I'd have to have an excellent relationship with my mum and feel she deserves it if I were to do it in the first place, I don't think your mum is in that position and she's being cheeky. If anything, now you have a baby and are on maternity, she should be insisting on paying or bringing stuff for the baby.