This is Bigal and before I retired I used to be a social worker (you can boo if you like)I worked with a whole range of peoples poblems - you could say from the cradle to the grave. Certainly one of the groups was people of limited intellgence and you cannot be sure how they will react to a given situation.
The information about this person is very limited -does he live alone, live with parents, live with a group, live with a partner. Are social services involved with him. The people he lives with might be very protective of him. He might show aggression very easily. Why was that particular allotment attacked? It might be jealousy, it might be personal, it might have random.
What I am saying is for gods sake do not try and tackle this alone or with a group. Do not confront him an any way. That is a job for the police who should be informed in the first place, and if necessary will contact a social worker who might know him. The other thing of course is how limited is limited intelligence? All that can play a part in a behavioural reaction, although high intelligence can still produce a vicious reaction.
PLEASE. PLEASE BE VERY CAREFUL HOW YOU DEAL WITH THIS AND LEAVE IT TO THE PROPER AUTHORITIES.