trees block line of sight, so if a monster is behind a tree, can the heros use blast on the tree square, and blast the monster at the same time.

I'd rule that you can't do it. How do they know the monster is there? If they do argue that you can, make sure you blast them with your creatures using the same exploit.

No, you can't do that. You can't trace LOS to the center of the tree space so you can't use it as a ranged target. However, hiding behind a tree doesn't help much. Simply target the space to the left or the right of the tree and the guy behind the tree WILL be in the blast radius. Getting on the tree space is a good spot since you can only be attacked by adjacent attacks then. And there's another way to prove your friend's trick is illegal. If I can't target a figure from range while he's in a tree, how can I target the space when he's NOT in the tree? Again, doesn't matter since you can fire to either side, but technically, your friend is wrong.