My take on the corrupting influence of The Coins:
0) If a character touches a coin, give them an appropriate aspect, like "Curse of the Blacked Denarius" or "Touched by a Fallen" or "Shadow of <Fallen's Name>" or some such. Either swap out an existing aspect (like getting an extreme consequence) or make it a "bonus" aspect. Getting rid of this aspect should be all but impossible. This aspect can be compelled as normal, granting Fate (or costing Fate to refuse). This is a very powerful (negative) aspect, but don't abuse this much at first; it's better to be subtle -- besides, keep in mind that each compel gives the character Fate, so the Fallen Angel should make any such compels important ones. A good time to use the compel would be when the character is out of Fate, and facing dire circumstances.
1) As to powers, the FA doesn't *force* the player to accept the power. He just makes it available, and encourages/tricks/etc the character into accepting. If the player refuses ... well, he doesn't get the power and the FA keeps trying, or bides its time until the character's need is greater.
2) The FA might be willing to offer a power temporarily as a free trial, or might accept some sort of bargain to do so. It's the FA's choice to offer the power as temporary, not the character's, and the FA would be offering the power to further its own cause. Basically, it's like offering a kid a free sample of a drug in the hopes of getting him hooked: "the first one's free!" In this case, use the temporary powers rules, and give the character Debt (unspecified future free compels). Use the Debt compels to get the character into a situation in which he needs more power. (Note that this includes access to Hellfire, as described in Sponsored Magic.)
3) Sooner or later, (possibly right away) the character is going to accept a power permanently -- either he forgot to negotiate it as a temporary power (with the deal ALWAYS defaulting to permanent unless specifically agreed otherwise), or the FA refuses to grant the power as a temp and the player is desperate. In this case, there are no take-backs -- at least, not by the character's choice. The FA may decide to allow the player to swap the power for another (generally bigger) power. Remember that for each power added to the sheet permanently, the character loses a Fate point ... or gains a Debt if he can't or won't pay.
4) Once the character starts using FA-granted powers (temporarily or permanently), he will start to change, in much the same way as described for Lawbreaking. That is, for each minor milestone reached, let the FA reword one of the character's aspects if he used an FA power since the last milestone. You can scale this based on how much power was used -- if a character accepted a minor temporary power but the bought off the compel with Fate, you might skip this step or make a very subtle change. If the character accepted Inhuman Strength permanently, make the change significant.
5) Once powers are accepted, setting down the ring will cause the character to lose access to the powers but not regain any refresh. Regaining the refresh might be possible, but should be very, very hard. At least as hard as getting rid of an extreme consequence. In addition, the character will still have that demonic aspect continually tempting him to take the ring back up ... and starving him of Fate if he resists.
Well, those are my ideas, at least to start. These are completely homebrew, so don't take them as Canon. But I think they fit.
In your particular case, if you and the player would like to continue with the character as a PC, you could do a minor re-write of what happened, inserting the above mechanics. Perhaps some of the powers accepted were permanent (at least some should be) while others were temporary. Figure out how much Debt he accumulated from both temporary and permanent powers (by your use of 'deep end', I'm guessing this will be high, perhaps ten or more), figure out what permanent powers he now has, give him the freebie demonic aspect and do a significant rewrite for one of his existing aspects (or maybe more than one if it seems appropriate; an example of a significant rewrite might be to change "Protective of my friends" to "Those who cross my friends PAY!"), and go from there.
