Willow had seen Violet's post on the net and was concerned for the younger girl. So she had decided to head to her work area to see if she could alleviate her worries somehow or at least lend a caring ear if she wanted to talk.
She knew it wouldn't be easy for people to accept Buffy's presence in the prison and that even those who hadn't been directly effected by Buffy's attack were still entitled to have worries or fears.
As the one who had performed the spell to return her best friend's soul to her Willow hoped she would be able to lend more strength to any explanation or reassurance.
She knocked on the door to alert Violet to her being there, since she'd heard the young woman talking to herself, "Hey, are you busy?" she asked with a kind smile.