ok imma hop in here, hi! i subscribe to you bc your art is hella fab my dude. so anYWAYS if it wasnt in their TOS before the sale, they cant do this legally. ya cant change your TOS to fuck over customers after a sale, because they had no chance to agree to the new TOS. unless stated otherwise, copyright and IP rights transfer via purchase, and unless the character was LEGALLY filed (press x to doubt amirite) she cant throw that word around to scare people. if she spent the money to copyright it, and then sold it, THEN forced the refund? maybe she'd have a case. but since she totally hasnt done that she cant threaten your friend and others with that. no copyright registration = no lawsuit, so she cant sue.
basically what shes done is bordering illegal and if your friend wants to, they can file a DMCA takedown, which forces deviantart to remove the image immediately. like. literally. it forces them to do it. its basically a cease and desist for the interwebs and if its ignored, the gateway for a lawsuit is opened. so obviously since DV doesnt wanna be sued over hosting someone elses shit, they'll take it down pretty quick. id recommend filing one, and it looks to me like DV has their own DMCA system that can be filed through the site itself, so it shouldnt be terribly difficult.
recently i delved into the world of copyright whatever so while im not a lawyer, i have read statements BY copyright and IP right lawyers and thats where most of this info is coming from. DV should side with your friend on this one. i like to use a house analogy to explain stuff since it really shows how outlandish the infringer is behaving. its like if galactic build and sold your friend a summer house, and then found out they werent living in it as frequently as they would have liked. this makes em mad, and so they force refund your friend for the house and start living in it, and then say "shoulda read my new TOS" when that TOS was not included in the contract your friend signed when they bought the house.
ya cant do that shit. its wrong, probably illegal, and certainly immoral. so thats my contribution, my recommendation is filing a DMCA takedown notice and see where that goes. best of luck, and i hope this ends in your friends victory!