Imagine someone was an alcoholic for many years. One day he hits bottom and asks God to heal of him of his alcoholism, because he does not want to drink alcohol anymore… Let’s now say this was a miraculous cure. He no longer wants to drink alcohol. So he no longer chooses to drink alcohol.
Was his free will compromised? Or when he asked for the healing, that entailed taking away the desire for alcohol and thus that one door was closed?
The answer is that his free will was not compromised. He made the choice to ask God to heal him. The healing entailed taking away his desire to drink alcohol.
In the same way, when an angel is created, God gives them a choice - him or the other guy. Full information is given. Then the angel chooses God, and is firmly in God’s camp and cannot switch sides. Is his free will compromised? No. He asked to join willingly and is given the grace to never turn away again. The angel wants God, and not the other guy. Because of that desire, the choice is made to never turn away.