It is absoulutely legal to give money in whatever amount as long as there is no criminality involved.
I worry that there are always unsubtantiated slurs about all sorts of things now nearly always years after the event and therfore almost impossible to prove or disprove
In this case the motives of the donor are unknown but receiving the dosh for a charity doesnt seem to be wrong to me.
Prince Charles is far from my favorite but we will be King.
But some seem to wish for a Republic. That can bring with it Presidents actually guilty of a crime (Sarkozy) or under scrutiny (Trump) or better still SADO CRIMINALS like Putin or Lushashenko.