how to prevent original owner from meeting your tenant

[Royal Bel]

I have a friend who has just started in real estate and he managed to rent a house then lease it to another and make profit from it. However, due to some unseen circumstances the first owner contacted his tenant and now they are ready to kick him out of the game.
What's your suggestion dealing with this situation? And how to prevent it from repeating?
Thanks everyone, you guys are the best.