Beautician voucher

[Peanut3]

I live in a small village and in the centre of that village is a beauticians. My friend and I recently celebrated our 40th birthdays and our very generous friends bought us each a £100 voucher for this beauticians.
When I went to use my voucher I discovered there was only £58 on it. Strange amount so I queried it with the owner. Turns out the other birthday girl may have turned up at shop for treatments without her voucher. The therapist probably asked what day was it bought on and subsequently took the amount due from my voucher in error. Upon returning to the shop with her voucher she probably discovered that it still had £100 on it and if she questioned it was probably told strange, but there’s £100 on it now. She probably didn’t question it any further - to be fair I wouldn’t have either.
So back to me discovering this today. Owner worked it out and told me that my friend owes me £42 and that I need to take it up with her. As far as I’m concerned it’s not her fault and I’m not even going to mention it to her. I was annoyed with the owner as they shouldn’t be taking money off vouchers that haven’t been presented.
All of us (including the owner of beauticians) have children in the same class at school. I would have thought that the owner should have held her hands up and said sorry our fault and honoured the full amount of my voucher as a gesture of goodwill.
AIBU?

I feel like it's there issue to sort as they made the mistake.

Definitely their mistake, therefore they should fix it!

I don't get why she didn't give you the other voucher that's on the system and the other lady the remainder from her previous treatment as it will show on the system she's had the value?

They made the mistake, their problem to fix.

You were given a £100 voucher, you have used nothing from your voucher, the salon owes you treatments to the value of £100.

Whether they chase your friend up about the use of her voucher is not your problem.

Why don't you just swap vouchers?

Thanks for all the replies. Our birthdays were in September, so by now the other birthday girl has had several treatments and her voucher is all used up. I only realised in December that there was a problem with the amount on mine, the therapist was supposed to look into it and call me back but never did. It was only this evening when I was there and asked the owner, that she looked into it properly and realised what happened!