How to pull off a one-off player escort mission?

[mortal mantle]

So the players are about to reach a really important session where they are about to rescue a crown prince from a dragons lair, but I was thinking of giving them a twist, and have the prince actually be controlled by a mutual friend of ours, who will be a guest for the session and the party will have to try and get them out of the lair alive

I can provide more details later on, but I got 2 weeks to prep for this session. Has anyone done something like this before and have any tips and recommendations to pull this through and make it fun for the whole group?

Ideally, I want to give the "prince character" an easy to control sheet with a couple of support habilities, with buff's and debuff's he can apply to the party and enemies.

Likewise, Im intent on giving him some more info about the lair, like the guards rotation cycle, secret passageways, as well as some other more important political tidbits which he can digress with the party as much as the player feels like it's relevant.

Ideally, they get a bunch of rewards by getting him out alive, and a lower, but still substantial recompense if they fail as "at least they tried".

• the loot on whatever is on the princes body as he is carrying a ton of valuable rings, jewels and bracelets, including an as of yet undefined magic item