Casserole with meat and dairy sat out 18 hours @room temperature. Can I boil it and still eat it?

Pitch it out!

I certainly wouldn't. Too much risk of toxins from bacteria that are not necessarily going to be destroyed by heat. Staph. aureus is a commonly found bacteria (it likely lives on your skin/hair/mucous membranes), but when in high protein food kept at room temperature it produces a heat stable toxin that causes typical food poisoning symptoms.

Please start over! 18 hours is too long in the danger zone, and it feels like boiling a casserole would ruin the dish. It's sad, but I think something else will have to be for dinner!