The Type 3 can show the 259 how to be more assertive and goal-oriented; how to stand up for oneself and make their dreams a reality.
Of course an unhealthy Type 3 could easily push the buttons of a 259 and harm them in that way; but that applies to all "Enneagram Duals", in one way or another. Someone who is the balance to your weaknesses could in the worst case scenario be your worst enemy. To some extent, that could even apply to "real" Socionics Duals who for some reason cannot stand each other.