A rather puzzling reaction; after all you wearing a kilt is just the same as her wearing jeans; its just a garment choice. Jess seems to be a true friend who can maybe get through to Sonya that your clothing choice just isn't an issue that should concern her. I think you need Jess and a few like minded friends to work on her and eventually convnce her that any problem with your kilts its entirely in her own mind. Meantime I wouldn't lose the valued friendship of Sonya, her sister and mother. I wonder if her sister and her mother perhaps have more empathy with your kilt wearing and may be able to persuade Sonya not to let it trouble her. As a general rule I'd say dress how you choose in what makes you feel comfortable, but it might be tactful to wear trousers to go to Sonya's party, rather than giving her the opportunity to make an issue of your attire at the party and make you the centre of attention rather than her. Just my two pence worth.