As far as we're aware, you/he probably has little recourse, unfortunately, since plant encroachment from one yard into another is not generally legally regulated in Maryland. (Refer to our 2018 blog post "What Can I Do About My Neighbor’s Plants Coming Onto My Property?" authored by an Extension legal specialist.) Having said that, a law recently passed in Maryland state government made it legal for local governments and/or municipalities to enact laws regarding bamboo spread. We have not heard about any such laws being finalized or taking effect, however, so you can reach out to county government to ask about the status of this. Information on our Containing and Removing Bamboo page might be useful with regards to getting rid of what is already present.

Miri