203 could be across the hall and/or downstairs from 204. It likely was not next door.
206 should be next door to 204.
At least that's how I've always seen addresses in Texas:
202----------
204 ----------
206 -----------208
--------sw------------sw-------------sw
201---------203 ------------205 -----------207
sw = stairwell. It appears each stairwell served four apartments.
If the numbers in the unit started with 201, then odd numbers would be on the first floor
It appeared Maria lived in building B, so I think 2 refers to the second building.
Maria Gonzalez texted her father that somebody was knocking on the front door of the apartment, but after that, the father couldn't reach her.
people.com
This apartment community was built in 1956 and has 2 stories with 32 units. (2 buildings...It looks like Maria lived in building B). It's possible that it was an old converted motel. Or just budget apartments that were built in 1956.
Either way, they were in close proximity, but based on numbering schemes that I'm familiar with, I doubt if he lived next door. It does appear they were connected with the same stairwell, whereas 206 was next door, but could have been connected to an adjacent stairwell.