Willingness of the protection against evil spell 5th

So we encountered a situation… a posess child. Wich I tried to do protection against evil… so the child could fight the posession by having advantage.

( old game already already had a dm ruling wich is fine)

But it made me question about the willingness of the target.

If he is already posession… obviously the spirit doesn’t agree to it. So how you get the advantage against the possession if you are already possessed.