How to deal with a player who showed his hands to influence the robber?
First off, yes, what he did was cheating. However, there's more details we need to know. Most importantly, did he know about this rule; and thus was cheating on purpose? Or did he not know, and it was an accident?

If it was on purpose, then the question I linked in comments probably covers it: How can I play games with closed information with players who cheat?

If it was an accident, then simply telling him about the rule so that he doesn't do it again is probably sufficient; at least for a non-tournament friendly game. The advantage that he gained was very small. Without cheating, he could have told Adam that he had nothing but sheep, and provided pretty good evidence of that fact by reminding him of what resources were given out over the past few turns. It seems more likely than not that the outcome would have been the same either way.

Ultimately, it's going to be a question of how you and your group generally handle mistakes during game play; not something that's specific to this scenario. For example, my group is very lenient about allowing players to take back / change their move if no one else has done something based on what they did yet. Some groups, however, may prefer strict rules enforcement over being casual.