Wendy will argue that his belief was not reasonable because she and Cartman are not only classmates but also friends and he should have known she would never actually harm him. Wendy could also argue that Cartman should have retreated before using deadly force, after all she was a little sauced. Cartman will argue that since the threat was one of death or serious bodily harm, his use of the little gun was, or reasonably appeared to be, necessary. The deadly force was not equivalent to the threat. Technically, consent is not a defense or a privilege.
Consent goes to the issue of whether there was a tort in the first place.
Keep certain principles in mind about the law of consent. An objective manifestation of consent negates a claim that otherwise would exist. Consent may be seen as going to the issue of intent. Peter, a third year law student, goes to see Dr. Joyce Brothers, a psychotherapist, for low grade depression and Chronic Fatigue Syndrome. After months of twice weekly therapy, Peter announces to Dr. Brothers that he is in love with her.