The same thing will happen, regardless of who she is standing next to. No one knows.
What we do know is that all the Phoenix has been doing lately is blowing up planets. Its stupid and villainous to attempt to sacrifice the world to "make a point" like Scott is so intent on doing. The only reason he isn't doing the logical thing and taking her off planet is because Cap thought of it first.
At the end of the day, Scott sees that if the earth gets destroyed, it would be worth it because he at least tried to restore mutantkind. He sees humans as pawns in his game. To him, if he's wrong, at least he tried.
Other then non-Jean going nuts, all other people who have hosted the Phoenix have controlled it.
So, the odds the Cyke is going by are good.
And he has Cable who told him about the Avengers (which came true).
Then he saw Hope use the Phoenix to heal Cable.
I'm a little drunk right now, so I'm forgetting something. Does Scott even know of the destructive path the Phoenix is taking? He knows it's coming, and tats it.