He will be her ally, considering some things. The reasons why are Victarion is something of a Chauvinist, but we can see that he actually loved the wife he killed, but is just too weak to admit it. And we see that he will do anything to get revenge on Euron, particularly using a woman, as a part of this weakness. Vic sees Dany as a opportunity to get his revenge ten-fold, and I think wants to marry her.

However, I don't think many people on their way to Dany, or who already with Dany, want Victarion around. And the horn is scary. And the dusky woman most likely is trying to kill him on Euron's orders, and Euron might already be planning some real shit and expected Victarion to betray him.

Then again, I think Victarion might have a long way to go in terms of his character, maybe improving as a human being. And also he may be a tool of lord of the light now, with Moqorro and his new charred hand (which I believe has given him supernatural strength), and may have supernatural protection that Euron may not have expected. So who knows. All I know is that if he lives and allies with Dany successfully, some other people gotta die.