By breaching a pact with a daemon, he's really playing with fire. Daemons don't like being double crossed or failed and if the player does it enough then he will be making a very powerful adversary. To be honest, corruption is nothing compared to what you could do.
What sort of daemon is he betraying may I ask? Is it a Khornate, Tzeentchian? Depending upon whom he does cross, we can help come up with suitable methods of long term retribution. Also what character class is he? What's his archetype and how is he playing his character?
By this for example, if he's angered a Tzeentch daemon and he's a stealthy character, the weavers of fate may conspire to make the light shine on him at just the wrong time to allow him to be seen and have that camera spot him when he thinks he's clear.
If he angers a servant of the Grandfather Nurgle, then foul toxins and plagues may soon find themselves befalling him. Disease and illness become more common and his weapons and armour end up becoming frail.
There are plenty of curses and rituals in Tome of Excess I think, which you could have used on him.