I would give them an aspect with a certain length of time (perhaps a session or scenario, or more if it was a payoff that could last that long) like, "Got The Big Pay-off" or "2 Million Dollar Man", where they can get a +2 to their rolls, or Invoke to be able to afford what makes sense.

At the same time you have to keep in account that his character choice is not detracting from the fun of the rest of the group. The needs of the many supersede the few after all.

its still the invasion of the mind though.  Unless Harry signed a contract when he signed up to the WC saying that he would let the Merlin enter his head anytime the merlin wished.

Transformation
and Disruption
Thaumaturgy that fundamentally, lastingly
changes the target—whether it’s the target’s
body, mind, emotions, or even luck—falls into
the category of transformation and disruption.
Often, this is dark stuff—curses, mind control,
destructive shapeshifting, and death magic.
Of all the methods available through thaumaturgy,
these are the ones most prone to run afoul
of the Laws of Magic (page 232). Regardless of
what the spell changes, this is a violent act to the
target: people and things are very good at being
what they are, and this sort of magic forces them
to be what they aren’t.
As such, these forms of thaumaturgy rely on
the same mechanical principle—most of them
inflict consequences or temporary aspects on a
target. Entropic curses inflict aspects that reflect
bad luck and other kinds of misfortune. Emotion
magic inflicts aspects related to emotional states
(lust, anger, fear, etc.) that the victim can fall
prey to. Mind control is just that—the aspect,
when compelled, forces the victim to act in a
certain way. In rarer cases, a curse might actually
be fully transformative, changing the shape
or nature of a being permanently.
Because these forms of thaumaturgy function
via consequences, a wizard needs to make
sure that the spell is complex enough to overcome
any resistance the target might be able to
raise (defense rolls, stress tracks, etc.), as well as
add enough shifts for the desired level of consequence
(0 for a temporary aspect, 2 for mild,
4 for moderate, 6 for severe, 8 for extreme).
Anything that is fully transformative must be
powerful enough to achieve a “taken out” result
on the target, which can be extremely complex
(see “Contests and Conflicts,” page 265)—which
isn’t to say there aren’t sorcerers out there practicing
that kind of black magic. Sadly, there
are plenty.

The whole 'free will' thing. You can talk to the willing dead (ectomancy) but not make them do anything (necromancy). You can't break into their minds (Molly, for example), but you can go in and look if given permission Turn Coat Spoilers (Gatekeeper and Listens to the Wind in Turn Coat).

Furthermore, this isn't turning them into a newt, and destroying their minds with the shock and instant brain-size change. This is changing their appearance. While it is an important part of your self-image, it doesn't change your body in those importantly dangerous ways that full Transformation would.

Sponsored Magic [-Varies]
Description: Some varieties of magic draw
on power sources external to the practitioner.
Invariably, these sources of power have some
kind of agenda of their own. See Spellcasting,
page 287, for details about the various kinds of
Sponsored Magic.
Notes: The cost of Sponsored Magic changes
depending on whether or not you already
have Evocation or Thaumaturgy. This also
affects whether or not Sponsored Magic gives
you any additional focus item slots. If you’re
paying full price, you get four focus item slots
with this ability. If you have either Evocation
or Thaumaturgy, thus reducing the cost of
Sponsored Magic, you only get two additional
focus item slots. If you have both, reducing the
cost more, you don’t get any additional focus
item slots.

Moriden, you don't have to break their nose or use blunt trauma when you have magic. Part of the spell could make it flexible, then you shape it, then harden it again. Besides, I doubt normal plastic surgery is anywhere near an extreme consequence. Dangerous, sure. But completely possible and I doubt it would cause anything more then a moderate consequence.