Rude manager

I purchased a swimming costume for my mum, and within 10 minutes she went to store to return it, however the manager not sure his name was on duty, refused for her to exchange the size and said she tampered with the costume, what can be done in 10 minutes with a swimming costume, He was extremely rude to the point where my mum actually left the costume and told him keep it as a Easter present cause it does not fit her and she is not getting anywhere with this conversation....