Bolins knows about what? About what Lin and Tenzin are up to with their digging around? I doubt he'd wanna tell Korra in case they told him NOT to tell Korra because this isn't really her fight (yet). Though it would also put a test to how much Bolin trusts Lin and Tenzin and how much he trusts Korra. Unless of course you mean something else heh. If anything, Lin could stay for tea and then head back. I just don't see her staying the night since she feels she's already said too much and is acting quite unlike herself enough as it is. She is slowly trying not to push people away but it's a habit that is going to be tough to break because she probably accepts shes like her mother in the sense that getting close to people doesn't work out for her.