I do think the other person is in the wrong – he was given the ad space in exchange for services as mod. Then he quit moderating – he should have notified the site owner and told them so and given the ad space back.
In any case, this is why I shy away from having partners. People have changes in life style, and you are stuck with twice the work and half the pay.
When you give away part of the profit the site makes, you have in effect a partner.