I took a bath yesterday and about 1/2 hour after I drained the tub, the basement tenant said his bathroom, hallway and closet floor were covered with water. Does this sound like a clog or vent issue? My boyfriends seems to think it's the septic tank but he tends to jump to the worse possible scenario first. We just cleaned the vent on the roof out last week, this was the first time the bath was used since. I'm thinking something got stuck, possibly from being pushed down further rather than pulled out of the vent. Thanks for the help!

This is a clog. To check the septic tank Have the BF locate the house cleanout and open the cover. Now flush a toilet a few times. If the water comes up out of the cleanout you have a septic tank that must be pumped.
If it starts to back up on the first floor locate the fixture and snake from there. If you don't know how to snake a fixture I can walk you through the process. Let me know what happens, good luck, Tom