When Sam picked up Cat they were facing directly towards the window, they should have seen Gwen & Ruby, though they were too busy fighting.
Sam and Cat shouldn't know exactly when to start pretending to fight if there was no way they could see Gwen and Ruby from their apartment.
When Sam was tricking Cat with the money, she gave Cat one ten, two fives, and one twenty, but then her hands were empty, which means Sam had 40 dollars, yet earlier she said they got paid 60 dollars.
You see Goomer trying to put together his severed meat patty, but Sam clearly only picked up the burger and dropped it.
Goomer has never met Gwen and Ruby, but he seems to know about them.