I don't think this is the case, no matter how much fuckery it turns out there was with Obama.
The legal problem, it seems, is that Congress certified the votes of the electoral college both times Obama was elected. From a legal standpoint, the place to challenge his legitimacy was there, or else to then impeach and convict him. But none of this ever happened, so I don't think there is any quick and easy "legal" resolution no matter what comes out about him. If someone wants to claim he wasn't "really" the president, then from a constitutional standpoint it would need to be one of the three branches of government that needs to say this, acting within its constitutional capacity, and I don't see this happening, no matter what anyone discovers. And it is probably the wrong way to go politically too.