Hello, I own a 2plex when my tenant fills her sink then makes it flow I have her water in my sink below.how can I resolve the problem.thank you.

Does it go down from your sink relatively fast or slow? Could be a partial block causing that amount of water to temp back into yours.

Just start, or continuous?

You are both using the same main drain line which is obviously clogging up down stream of your sink. Rod that drain line starting at your sink right after the trap. Do you have a main drain clean out cap somewhere?