Then i would suggest to re-write the description: Target ally and some others come to assist. Then the target assistant and the Ghost gain stealth. This way it would be obvious that the target does not classify as assistant (bc the assist never occured) thus it shouldnt gain stealth either.
Problem is having 2 in stealth in a 3v3 opening is a soft forced taunt for obvious reasons, and whereas the rebel fleet does not require a powerup, i think this is misleading and the effect is weaker than the description.