Well, hard to say. The kid does look pretty depressed since he's sitting hunched in a dark corner for whatever reason. But the look on her face tells me the "comfort" she's gonna provide him is gonna be over the PG-13 rating.
Really a "Speculation Fuel" material here. There are possibilites: - Sonico kidnaps him and locked him in. Yandere-style. (If you read it from left to right)
- The boy got locked outside and Sonico offered him to stay on her place for a while. (If you read it from right to left)