he is a man.
Yara has a fleet, is going to get there first, and as noted, just in time when Dany needs a fleet to fight off the Master's fleet. Now dragons vs masters fleet does seem predictable. But as noted, if dragons burn the fleet, Dany still needs ships. Yara.
so, where does Euron fit in?
Dany is almost certainly going to accept Yara, and their fleet. Euron shows up too late, and what ?offers his fleet if she marries him? She has been forced to make some actual diplomatic choices, but that is laughable.
So, either he murders Yara to claim all the fleets. or, the dragon horn thing suddenly shows up, and its a total shitshow. Dragons fighting dragons, both fleets burning. or, they just walk right over Euron and his fleet, making him pointless, save an excuse to push yara/Theon to Mareen and Dany.