They make mattress covers that zip all around the mattress and the box spring and keep bed bugs trapped inside. They're made just for keeping bed bugs away.
That's not the issue, though. The complex is obviously infected and unless the entire place is treated and all the tenants manage to take precautions with their clothing and vehicles, there's no way they're all gone. Badly infested places will have the bugs drop down from the ceilings. The complex owner would have to do treatments for the building multiple times to eradicate them all. They're much easier to defeat in a single family home than in an apartment complex.
Heat is much more effective, but freezing at 0f (normal temp for a household freezer) will kill bed bugs and their eggs. Problem is that it needs to be done for at least 4 days. That's why heat is better. Heat will kill them much faster.