From the sounds of it, you've got two characters feeding most of their wealth into buffing up a third character. That that third character has now become considerably more powerful is only a logical consequence.

If the players have a problem, it's their own fault. Logical answers are
Short-term: the players consider selling off some of the loot they've overclocked their friend with.
Long-term: Put up with it for the moment, but give the Lizard a significantly reduced share of treasure so that the others can catch up in terms of wealth worn.

If the players don't have a problem, I'd go with the option of throwing one big 'un and a group of minions at the players, so all characters have something they can reasonably face off against.