Lisa,lying on the bed, pleasuring herself

There is her friend Lisa, lying on the bed, her right hand is between her legs and under her skirt. She is clearly masturbating. She is moaning and her body is convulsing and contorting. Her left hand is squezing her left breast while she gets herself off.

Embarrassed, Katie isn't sure she wants to disturb her friend, yet an odd erotic feeling has come over her and she is turned on by what she sees.

Hmm, she thinks to herself....should she leave Lisa alone,or see if she can join in?
Or maybe she should go into another room and relieve her own sexual tension?