The law hinges entirely on the term "expectation of privacy" so what you do you mean by "somewhere private"? You mention a sofa so if your friend invited you in somewhere like the living room (ie. no bathrooms, weren't filming his naked kids, etc.), my understanding is you would be fine. Your friend might not like it, but legally similar situations happen daily, it's called Facebook.

Imagine taking a picture at a party with intoxicated people in it, do you get everyone's consent before snapping the pic? What about being filmed on security cam driving past an ATM? So those are two examples of where although you might not actively expect to be filmed, they are still protected filming locations.

edit: typos