In the series - and in most approaches I can envision to the game - gaining or losing powers is a big deal. It won't be occurring every two seconds.
I suggest that you tie this in to Fate's normal system for gaining, changing and losing powers - milestones.
Your NPC simply has a trapping "Power Transfer" which is a plot-enabler - it constitutes an excuse for the character being able to gain, lose or change a power at a milestone. If you want you can then let the change happen mid-play according to the events of the game, with the refresh and mechanics being adjusted at the next milestone to match.
If you do want powers to transfer frequently, you need to look at the entire system, because that's a major change to the style of the game. (Potentially a very interesting one!)
Mechanical balance of powers is actually a bit less important, though, if nobody's going to hang on to them.
So: Losing powers, or trading for lower-refresh cost, is obviously not a problem. Give the player back the difference in refresh at end-of-session, and you're done.
Gaining powers, or trading for higher-cost ones, could easily be implemented using the approach from Dresden Files: if you have the refresh to pay for it, you just lose the refresh (and fate points) immediately.
And if you don't, Really Bad Things happen. In Dresden your character falls to supernatural lack-of-self-control, and that's a theme that fits really well with Misfits - minus the supernatural.
So: a character's refresh represents the maximum amount of power they're stable enough to handle. If their refresh is reduced to 0 - or below - by extra powers, they become rapidly unstable, violent, sociopathic or otherwise relegated to dangerous-threat-NPC territory. (There are several good examples of this in the show - Tony, Brian, arguably Simon, and more.)
I suggest the player gains an extra Consequence reflecting their deteriorating sanity, and then has until the next milestone to either gain more refresh, or reduce the cost of their powers - before they become an enemy NPC.