The sesh/session here refers to a period where 1 or more people are smoking marijuana.
Caroline: Really? DFAS to cheer you up?
Savannah: LETS GET DRUNK AND PARTY!
Paige: Na, I don't want a hangover , I'm working tomorrow.
Savannah: Oh, DFAS instead?
Paige: FUCK YES!
When Lemmy Kilmister from Motorhead soundchecks, the sound engineers always ask if the "DFA levels are OK." Motorhead shows are well known for being insanely loud and fast, and so the DFA term has been inside joke for Motorhead for many years.