The whole thing hinges on whether she had capacity. Capacity related to the decision being msde, it isn't an all or nothing situation. A person can have capacity to make a decision to buy a new coat but may lack the capacity to weigh up the pros and cons of a financial decision of more magnitude. So, was she able to understand the decision, and weigh up the pros and cons of the decision? If she knew full well what the bad consequences would be and understood the likely result, then she was entitled to make an unwise decision but if she didn't understand the ramifications she did not have capacity for that decision. You are going to need evidence and that might be difficult. Is there anyone who can provide a signed statement in support of the contention that she did not fully understand the implications of the decision? A GP, bank manager, anyone who has has dealings with her who might know that she was unable to weigh up the merits of deciding to make this large donation? If she has a written diagnosis of dementia and you have a copy, that might be powerful evidence. The Charity commission is the regulator for charities and you could try complaining to them although they are rather spineless. But first gather evidence, if you can, to convince the charity that she did not have capacity to donate the dosh.