Actually, you can't change the definition of ally/enemy in the middle of resolution of a power.
Let's say Chuck is pushing Bob through Alice, and Alice is Bob's ally.
The reason is Bob is not using a power on Alice, so Alice cannot make a decision on whether she is a willing recipient or not of his power. The only thing Alice can make a decision on is Chuck's power, and whether she is willing or not to get hit by Chuck's power is irrelevant to whether Bob is an ally to her. What matters is Bob's powers.
So, every time Bob uses his powers, Alice is willing. And therefore, Alice is an ally. Alice does not get to decide whether or not she is an ally, being an ally is a state defined by her willingness towards Bob's powers. Until Bob uses another power on her, her willingness remains the same.
'So, Bob used healing word on you. So you're an ally.'
'But I'm an enemy now cause of what Chuck is doing.'
'I see. So, checking your willingness... Bob used healing word on you, and you were willing. So you're an ally.'
'But I want to be an enemy!'
'Then you need to start saying no to Bob's powers.'
Being an ally is predicated on player action, and as such, the DM makes the ruling accordingly. The player does not get to decide if they are allies or not, they decide if they are willing or not, and the DM rules accordingly.
Defiling is simply badly written and they just should find a better wording to accomplish their goal.
Defiling contradicts the allies rule. Specific beats general. It's not hard to understand. Current allies can make themselves unwilling but it does not affect how Defiling works on them. They remain allies for the purpose of defiling. Thusly, they cannot evade defiling by becoming 'unwilling'.