Scripture says plainly, the hebrews could only posses the land if they were obedient to God. If they were not, they would be cast out of it.
Could just drop him a few verses of the bible to put the issue to rest. It really is that simple.
Here is a jew saying the same thing if a link would help.
https://www.rt.com/op-ed/524455-neturei-karta-rabbi-weiss-israel/
(post is archived)