I'm looking for other opinions on this as I think I handled the situation in a reasonable way, but DSD (after a discussion with her mum) has said I was very, very wrong and she's so upset she can't be around me anymore. Apologies for writing such an essay!
For a bit of context DSD15 lives with us and stays with mum every other weekend. Same court-ordered routine for around 6 years. Prior to 'the incident' I would say we got on pretty well, definitely better than I did with my parents at 15!
It started because one of my friends was upset she couldn't afford a laptop for her son. I said I could help as DSD has an old one she hasn't touched in almost a year. I was sure she'd want to help and she knows my friend quite well. I went to ask DSD, told her about the situation, and explained that I could transfer any old files and reformat the laptop so he could borrow it for a few weeks. She said no. A flat out "no" and would not explain why.
I found this quite upsetting as it had never occurred to me that DSD would not want to help; she's got quite strong morals which she enjoys discussing. Later that evening she asked me why I had been so upset, I explained that I felt bad for not being able to help my friend and I didn't understand why she didn't want to help. She didn't offer any explanation but we hugged, she kept her old laptop, and I thought it was done. However, after DSD talked to her mum about it (they made notes together so DSD could explain her thoughts 'correctly'), DSD has said she's extremely upset about how I acted and can't be around me because I stress her out too much. She thinks I should have accepted her "no" because the laptop is her property. But mainly I was wrong to tell her why I was upset because that is an "adult conversation" and not suitable for a child to hear. However, I think at 15 DSD is mature enough to hear/see how her actions affect people, especially if she asks about it.
AIBU? I suspect DSD is feeling bad for not helping and also feeling stressed by school, but instead of addressing the root causes DSD and her mum have made me the scapegoat for all her negative feelings. I feel like I did the right thing, but I keep second-guessing myself. My partner, DSDs dad, agrees with me completely but some outside opinions would be really helpful.