Here is an idea.
Theives cannot pick while victim is in attack mode. And vice versa, theif can pick pocket while the theif is in attack mode.
On the other hand it will stick to realism if the victim could be pick pocketed at any time outside of city gates while not engaged in battle.
If they are still in the city and in plain view of a guard, the guard would subdue or attack the thief. Other side, if they are in the city but in a non-populated area... aka alley or out of sight in a \'shady\' area, the guards cannot see and would not get involved.
Victim should have a chance to get stolen goods back by attacking or subdoing the theif.