(It's long so there is a TL:DR recap at bottom)
I bought discounted tickets (on offer) several months ago for an attraction, the tickets were for specific day (a Friday).
Three weeks before something happened at work and I was no longer able to take the day off, I said I would just sell the tickets and go on the Saturday (the next day) instead, my friend, S, asked if she could buy the tickets.
I said she could have the tickets for free if she would buy me 'return tomorrow' tickets from inside the attraction (RT tickets are slightly cheaper than my discounted tickets so it was a really good deal for her)
She agreed and I gave her my tickets.
The week before I checked that she was still going and our agreement was still on and she said she wasn't sure, she had to check her mum could pick her kids up from school, I asked why she hadn't arranged that already and she said she forgot.
I had a feeling it was just an excuse she wasn't really sure about going, it's a three hour drive away so I told her if she didn't want to go it's fine, I could easily sell the tickets or offer them to another friend who had shown interest in our arrangement if S changed her mind.
She thought about it overnight and then said that she did definitely want to go and her mum was fine having the kids.
The day before, on the Thursday morning I saw her at school drop off and I checked she was still definitely going, she said she was but then late that night she texted me and said that she wasn't able to go but her sister was going to go, use the tickets and get me RT tickets, I was a bit 
but said as long as I definitely get RT tickets I don't mind.
She texted me in the morning said her sister had gone and she arranged for her sister to drop the RT tickets at my house when she got home around 8pm.
8pm came and went and at 10pm I called S to see where her sister was, she said she'd call her and see.
There were several calls back and forth, me asking questions, S calling her sister and then calling me back.
The cliffsnotes version is the sister said that the discounted tickets didn't scan at the turnstiles and they had to go to the counter and buy full price tickets to get in.
I asked what the attendant/staff member said about the tickets, why they wouldn't scan and she said "they didn't say anything, she didn't ask about them she just thought they wouldn't work so bought new tickets"
When asked again she confirmed she didn't ask for help when they wouldn't scan she just went to the counter and paid full price for new tickets, she did show the tickets to the person on the counter either.
I was apologetic and asked about my RT tickets, I said I'd pay her for the RT tickets but she said her sister didn't get me any RT tickets as she was so annoyed about having to spend a fortune on full price tickets.
S said she was really annoyed with her sister for letting me down (about the RT tickets) and said she'd pay for my tickets to go the next day, she'd looked online and you can get a % off if you book online and she'd give me back the money when she gets paid on Monday.
She told me to get tickets and let her know how much it was.
At first I didn't want to take her money but after thinking about it I don't think I should be out of pocket either, I already bought the discounted tickets which have now gone to waste and I've had to spend twice the cost of those buying tickets to go on the day we planned.
I also don't believe the tickets I had wouldn't have worked, they were bought on the attractions official website so were definitely 100% genuine tickets.
I think it's more likely they printed them out in poor quality and the barcode just wasn't scannable, but why would they not bother asking for help, I don't understand why her sister would just pay over £100 for new tickets instead of trying to find out why the ones she had weren't working, doesn't make sense.
I'm really torn,
WIBU to let her pay for the tickets I had to buy?
TL:DR recap
I bought discounted tickets to an attraction but couldn't go.
Friend (S) said she would go and get me return tomorrow (RT) tickets.
We arranged it all the S seemed to no longer want to go, I offered to give the tickets to someone else or sell them but she insisted she did want to go.
The night before she called and said she couldn't go but her sister would go instead.
Her sister went but said the tickets didn't scan, but she didn't ask for help or show them to a member of staff at the attraction, she just paid full price to get in and she was annoyed about that so didn't get me the RT tickets.
Since i had arranged to go the following day S offered to pay for the tickets I'd had to buy to go.
WIBU to let her?