In ROP:Faerie a faerie's body is comprised of incidental matter.
In Mystery Cults when a magus is on the Path of Becoming and gets a Faerie Body they stop aging and become immune to deprivation, but do their bodies become incidental matter? What happens if they try and transform the spirit before the body so they have might in a human form? If a human with a faerie body is killed do they reform like a faerie would or stay dead?
If they transform the Spirit before the Body, you get Might in a human body, but seeing as you stop being able to advance your Hermetic Arts and get them replaced by powers you probably want to transform your body and mind to match as soon as possible.
Seeing as Becoming:Spirit is the hardest part (you need to match the total of all your Arts added together, but your total consists of best Technique+Form+Int+MT+Faerie Magic+aura, so if you have the 5 in the other 13 Arts to train an apprentice you need to get 65 points from somewhere, so the 60 point bonus for a major flaw is pretty much compulsory), it's likely this will be left to last. Not always, sometimes people get the numbers wrong and get stuck as an ageless mage who doesn't sleep and can't gain new abilities.
If the person dies having become a Faerie...1) According to page 10 of RoP:F a Faerie who gets catastrophically disrupted can't remove their vis in time, so if someone (eg a wizard they've upset) grabs the vis and uses it they could be toast
2) When Faerie come back, it's not always as the same Faerie corresponding to the exact same role - so you could be justified in either saying the character now becomes an NPC faerie (if you want parity with other players and "no resurrection without a divine miracle") or in rearranging the characters personality and any Story Flaws (because their role has changed slightly - works if the player is OK with character redefinition as a consequence of losing their body)