[Rocket Man]

Brainwashing tends to depend on two things:

1) Isolation, so that others can't contradict the views you're trying to inculcate.

2) Conditioning, a steady encouragement of "good" behaviors and discouragement of "bad" ones.

If the character is already part of a PC party, then number 1 is in a great deal of danger and your ability to provide number 2 is jeopardized. Maybe not impossible, but difficult.

One of the keys to this from Hell's side is that the target must learn to mistrust the Seraphim he knows. Give him reasons to dislike them, avoid them, consider them narrow-minded killjoys, or beings so focused on universal truths that they don't understand common realities and necessities. Make relationships with them cool or even chilly; if he's rude enough, the Seraphim may start avoiding him as unpleasant to be around. Note that this doesn't work so well with Seraphim of Dominic ....

Another thing I would recommend is working through mortals who are already on Hell's side. Get them to be his friends, because people are often predisposed to agree with their friends. Maybe even set up a situation where the character can get one of his "friends" out of trouble caused by narrow-minded zealotry, or by some group known to have Heaven's backing in the campaign. (Why mortals? Because if anyone checks to see if he's had commerce with demons, even a CD6 won't say "Yes"; also because mortals can have confusing aims and morals that can lead to celestials not understanding their true intentions right away.)

The trick, as you say, is to keep him from Falling or Jumping. So encourage minor things that are still part of his angelic nature ... an Ofanite might get the chance to go to the gym with his buddies on a regular basis, for instance, or a Cherub to walk that nice young woman home. But constantly, quietly reinforce the inner voice that tells him that even though most of his Heavenly buds are real nice guys (except for those #$% Seraphim), they just don't understand how the real world works and trying to explain would just get him accused of heresy. Thankfully, these guys understand ... and if he just slips them a little quiet help every now and then when they need it, no one Above really needs to know, right?

(The comparison may sound ghoulish, but it's rather like "grooming" a victim for abuse in some ways. Studying those sorts of cases may give you more ideas than you would want!)