Legal Question in Real Estate Law in Massachusetts

First, I suggest you calmly discuss the problem with your neighbor.

Assuming that does not work, then take pictures of the damage and then speak to the town building department and see if they can assist. Assuming the town cannot help you, then call an attorney in your area to bring a nuisance action against your neighbor for damages to your property. I would get an estimate from at least two companies for the cost of the repairs.

Assuming your neighbor has insurance on his property the insurer will defend and probably if the cost is not too much cover the cost of the repair,

Your neighbor will have to take action to control the growth of the Bamboo.