There is no point of opening the door if the guard is there... guarding.
How to get rid of him?
There isn't really any items, either.
Maybe talking is the way. Talk to the guard. Talk to him about Winfriede. Then talk to him how he doesn't like it here.
Did Reynard come?
Talk to Reynard about the guard. Say that he wants to quit. Reynard gets rid of him. Remember, you can use Reynard's help later.