my toilet water level goes down when my neighbor flushes her toilet
What's happening is that when they flush, it is creating a vacuum and sucking the water out of your bowl in an attempt to get air. The vent stack is almost certainly obstructed above your toilet's tie-in.

As far as what to do... you are in a condo, there should be owner's association that handles shared infrastructure. If not, find out what is blocking the vent stack and remove it.