2 of our neighbors have placed the hedge on the boundary line, each trims their own,
now a dog has joined and a small fence in front of the hedge so that the dog does not run through the hedge,
everyone is happy
We have agreed on the same with one neighbor, but legally it is not clear-cut with us either. Normally, whoever plants the hedge should keep 50 cm away from the property boundary. As long as the neighbors agree, no problem, then you just put the hedge exactly on the boundary line. It becomes problematic if there is ever a conflict with the neighbor or if one of them sells the house.
In the present case of the OP, it seems there is not 100% agreement about the type and form of the property boundary.
If the neighbor wants to put the fence on the property line and also pays for it, then he has the right to do so. If the OP wants a hedge, then with 50 cm distance.