The alternative approach is to view this from a roleplaying perspective. Do the other players' characters notice this behavior? How do they feel about it?
I could see this turning into a great chance for RP, and while it may split the party (or even cause some Player vs Player combat), it could turn out to be a really positive experience.
I foresee characters noticing the actions of the Summoner, or noticing that they have lots of shiny new equipment from an unexplained source. At this point, characters could feel jealousy, and demand that the loot is shared out in an equitable manner.
If the summoner disagrees, the characters could attempt to reason with them, or attempt to force the issue by subduing the summoner and claiming some of their loot for their own. If the behavior is continued, there could even be a party split, with the summoner being cast out of the party (I would only do this after discussing with all players, perhaps talking to the summoner on their own). If the summoner agrees, you could even arrange to take control of the summoner when they are cast out, and turn them into a recurring bad guy in the future.
Ultimately, at least one person is going to be unhappy at the end of this incident. You can choose to deal with it completely out of character, with the players, or you can encourage them to find an in-game solution, with all the ramifications which may arise.