Assault cases are Government vs Defendant, not Claimant vs Respondent as in fraud cases, so Dennis would merely have been a witness rather than a claimant.
The quantity of mushrooms the characters eat is nowhere near enough to have any kind of effect. Andrea tells Dennis to eat "just a bit of one" - a single desiccated psilocybe cubensis mushroom (the most commonly available variety) weighs less than 0.1 grams on average and a typical, low, dosage is around 2 grams. There is no known psychoactive mushroom with active effects at the quantity they consume.