What payment method is she trying to use to withdraw? Sometimes cryptocurrency casinos will allow you to deposit through a traditional method like Visa/PayPal but you have to withdraw to a cryptocurrency wallet, so she could try doing that. I would contact the support team via live chat and email and ask very clearly which method she is allowed to use to get the funds. Alternatively ask them if they can help you to put through the withdrawal request to a more suitable method.
Unless they ask I wouldn't draw attention to the fact that she is a UK player. Answer honestly if they do ask, but they might just close the account and say they don't accept UK players (which they shouldn't).
Try contacting the support team via email - support @ mystake.email (take the spaces out).
If they still don't cooperate you could try an Alternative Dispute Resolution service by contacting complaints @ curacao-egaming.com (again remove spaces) but I don't think you would get very far with them. It's always worth trying though.
Unfortunately I can't be of any more help as I've never dealt with this casino. I hope she does get sorted though.