When I come across this situation, I always seem to jump to the worst conclusion first. Then as I uncover the facts, I am better able to determine the client's intention. Most clients, I find, dont' really want to cheat, they just don't understand the connection of everything and the ramifications of it all. As there adviser, I inform them and from that conversaton I assess whether they are going to be a client going forward or not.
As for your situation, I think your stance is proper until you find out the facts and if it turns out that the income is under reported, corrections or walking papers are in order.