A player is fighting with a one-handed weapon and no weapon in the off hand. He wants to make a disarm attempt with his unarmed off hand. He does have the Improved Disarm feat but not the Improved Unarmed Strike feat (and is no monk).

I claim, that he provokes an attack of opportunity for making an unarmed strike against an opponent. I interpret disarm as an attack.

He claims, that he does not provoke an attack of opportunity for three reasons. A disarm is not an attack as such, but an attack-equivalent action, which he simply interprets as a standard action. Secondly he claims that Improved Disarm overrules unarmed strike rules in this particular case. Thirdly he considers himself armed, because he has a melee weapon in his primary hand.

So here are the rules:

• For disarming: As a melee attack, you may attempt to disarm your opponent (...) You provoke an attack of opportunity from the target you are trying to disarm. (If you have the Improved Disarm feat, you don’t incur an attack of opportunity for making a disarm attempt.) If the defender’s attack of opportunity deals any damage, your disarm attempt fails. (PHB 155)

• For unarmed attacks: Attacking unarmed provokes an attack of opportunity from the character you attack, provided she is armed. (PHB 139)

So who is right?