"... Ashley van Haeften also refused to resign ..."
It's not a good idea to appoint someone without a process being in place to 'disappoint' them.
"... and the charity told us that it isn’t responsible for the actions of its employees."
I thought every organisation was responsible for the actions of its employees, if they were engaged in the activity for which they are employed. Editing Wikipedia articles and using Wikipedia tools sounds like he was working. Wikipedia should have adequate procedures in place to enable anyone affected by a 'rogue' employee to have malicious articles and actions investigated and withdrawn.