Years ago I read a story of a similar problem with a neighbor. The answer then had to do with the fact that putting up a barrier–like a sheet–between the two properties trained the bees to go over the top of the barrier–higher than the neighbor was tall. After several days of flying weather–the sheet was removed but the bees continued to follow the same flight path. For the entire year!! I know this doesn’t keep the bees from being poisoned when visiting the neighbor’s tree.
Good luck.

we beekeepers have a saying: If your neighbor has a problem with your bees….you had a problem with your neighbor before you got your bees….

Inform your neighbor that, while the tree is mostly self fertile, bees will increase the fruit production, and he might as well remove the mango tree, as why bother having it if he doesn’t Want more mangoes(?). Research showed that 50 different insects, including your native bees, flies, and honey bees, come into contact with the reproductive parts of the flowers. Inquire as to how he’s going to So selective of the pollinators, and if he sprays poison on his tree, the poison will become part of the fruit, build up over time inside his precious granddaughter, and cause health problems down the road. You could tell him that if his granddaughter talks to the bees while she is playing in the yard near the tree, that the bees might bonk her, that as long as she doesn’t try to pick up the bees, most likely the bees won’t hurt her, unless they are trying to tell her something is wrong with her health, and may sacrifice themselves to point out or try to heal her. Maybe introduce her to a honey bee drone(no stinger). Provide some info about apitherapy, to back up the healing angle. I have a neighbor that was kinda freaking out in conversation, when I mentioned that I’d be raising bees, but even when our fence was down(windstorm damaged, fixed now) and the hive was occupied & visible, never was there any hue & cry raised. What most people don’t know, is swarms are mostly quite calm while looking for a new place to set up hive. Just the bulk of a swarm and possibility of Africanized bees, is frightening. I’ve only read of that problem in the Western hemisphere(Africanized bees). Hope this helps.