Apartment next door running my tenants off

[PJ Muilenburg]

I  bought a duplex last year in a lower income neighborhood. Good tenants but one told me he's planning on leaving due to the small apartment (12 unit) getting bad next door. My duplex shares a driveway with the apartment so they're pretty close. The neighbors' friends are sleeping in their cars in the parking lot, people are coming and going during the night, noisy during the night, using the water hoses to bathe. He also suspects drugs are being used but hasn't seen it directly, only heard them talk of it. He won't call the police himself because he's afraid they'll mention his name.

How would you recommend me handle this? I do have the landlords contact info and I know he has 80+ properties. I do plan on contact him to see if he will resolve but he's blown me off before about something. What's the best way to get police involved?