score:3
A ban is probably legally possible but very unlikely. I assume a fine is also possible but not certain. With a bit of luck, they might also get away with a slap on the wrist. It could however become more difficult to secure another visa in the future.
But the consequences of the overstay is not the only thing you have to worry about: Border guards are perfectly entitled to examine your plans upon entering. If they notice the discrepancy, they can also revoke the visa and send your friends back to Thailand on the grounds that they don't have proper documentation for their intended stay. That would be much worse on every level.