I have been thinking that something of that sort probably happened. Adrenaline + blinded by either his own flashlight or possibly some sort of a light being on in Xana's room (she at least was awake) or even the Good Vibes sign or similar.
As for the light BF thought she saw, her bedroom was on the lowest floor, one floor below the living room and Xana's and D's rooms - most of the effects of any struggle should not have been visible to her. However, the stairs that go down are right next to Xana's room. On the right hand side, while the bathroom is on the left hand side before her room. Still, the stairwell also has a full-length barrier wall in the living room, that seems to be thigh-high.
On the lowest floor, both of the doors are right next to the stairs, one facing the going-up and the other facing the going-down direction. While it is possible that if any light was temporarily flashed in the downwards direction, it could have illuminated the floor behind her door, it grows increasingly unlikely, if the stairs are fully separated from the hallway with a wall - I am having a hard time finding actual photos of the hallway on the lowest floor. I am not 100% but I believe BF was in the room under the living room part, not under Xana's room. And after all, seeing "a flash" is the easiest thing to be mistaken about. When there is lightning i often go "was there a flash just now or did I just blink weirdly?" in my head.