Find a way to touch their gold, without them knowing or looking, after casting Aristocrat's Nightmare on yourself. Bye-bye all his/her gold, it is now only chump change until it is all, slowly and painfully, changed back to gold with remove curse in batches of 50 per cast.
For something with bite, talk to your DM on just how creative you can get with Bestow Curse. A very fitting one out of the Horror Adventures book would be that prevents him from healing naturally thus making him almost entirely reliant on you to keep him from dying death by a thousand cuts.
Cast puzzle box on everything they own.
If they have a love intrest mess it up. Use disguise self or polymorph to turn into random pretty girls to walk up and flirt or say things like "had a great time the other night". Disguise yourself as a mailman and claim to be returning a letter. Have the letter be a love letter to a woman who supposedly lived(or even existed at all) at a recently abandoned house
If he owns a building casually ask if you can move a few things around. If he says yes cast Renovation ^__^
Planar binding a bunch of really annoying angels/azatas. Tell them about what happened and command them to act as his conscious for the next few weeks.
Set up your own fake questline. Planar bind an Ifrit to stay in a lamp until "Insert party member name here" rubbs on it then grant him a twisted wish. Then drop the lamp off as loot for your fake quest and throw out a plothook. Probably might want to ask gm about this one due to time consumption.
Anything he is proud of owning anything, outdo him. If he's proud of his warship buy a bigger one and park it right next to his. If he has a cool magic item go get a better version
Pick up conditional spell. From now on any of his healing and buff spells come with conditions. Feel free to consult an asmodian for condition ideas.
Next time you are going adventuring use treacherous teleport to get there
Suggestion/bestow curse- end every sentence with "as the prophecy foretold"
Start/promote a religion/organization that is in opposition to the PC's beliefs. Bonus points if you volunteer them for related charity work that they would publicly look like a complete jerk for "backing out of"
Step 1: Next time you're in a tavern, pay a thiefy-roguey type to feign sexual interest in that PC and drug his drink. When he wakes up, he's tied up naked to the town flagpole.
Step 2: While he's knocked out, purchase a pony and attach his bag and all his personal effects to the pony (making sure all his bags are open). Also, purchase a Scarlet and Green Cabochon Ioun Stone 10,000gp (Endurance feat) and give it to the pony. Take that pony to the edge of town and slap the pony's butt and yell "Hyah!", causing it to run through the forest or plains or mountains (wherever you are) and the PC's stuff falls out at irregular intervals (with Endurance so he won't get tired).
Step 3: Wait for the PC to wake up and tell him if he can find the horse, he get's 10,000gp, and of course mock him with his current situation. Meanwhile, go pour yourself a mug of ale IRL for "victory sips" as you watch the PC make d20 roll after d20 roll to find all his things (while naked). For added insult, also hire a witch to cast Scar hex, Misfortune and Cackle while he looks for his things (at least within the first mile). "awww... you rolled another 1... aww T_T <victory sip>"