Dealing with neighbor of a tenant

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Hi all,
I have an issue with a neighbor of one of our tenants. He has butted heads with our tenants ever since they moved in a couple of months ago. He has called the cops on them twice. He has accused them of convincing his tennant (he owns the duplex next door) to move out by telling them bad things about him. The police confirmed that they moved because of a job opportunity.

The tenants are not the issue. They are good people and have tried everything to difuse the situation. Now he has filed a lawsuit with the city for tresspassing! The only thing the tenants can think of is that they accidentally mowed over the line a few days ago.

Should I step in and get involved in this? I don't want him to cause them to move. If he does, is there anything legally that I can do if he does?

Thanks,

Eric
Peoria, IL