Please remember that you very likely are hearing only your friend’s side in this dispute and that people who are terminated generally are not able to be completely unbiased in their reporting of the events leading up to the termination. What this means is that the priest may have had just reasons for needing to make a change, reasons that your friend apparently cannot and the priest will not be able to explain to you. While your friend deserves sympathy and appropriate support, your priest deserves the benefit of the doubt.
You also have a responsibility not to cause scandal for your wife and children. You say your convert wife is “disappointed” in the Church, and your children very likely are young enough not to be able to appropriately distinguish between loving sinners and hating sins. That means that you as a Catholic have a moral obligation to model for them appropriate respect and love for Catholic clergy, especially those in legitimate direct authority over you. If that means swallowing down some indignation, perhaps you could offer up the suffering involved in doing so for the needs of your friend, this priest, and your family.
It is not wrong to leave a parish when just reasons make it necessary to do so. But I ask you to carefully reconsider whether or not just reasons really exist in this instance, especially given the scandal for your wife and children that will necessarily follow.