The person who interrupts gossip about an absent friend is braver than they get credit for.

Gossip runs on agreement, and the easiest thing in the world is to let it keep moving. Stepping in makes you the odd one out for a second and turns the attention onto you, which is the whole reason most people just look at their phone instead. You do not have to make a speech about it either. Changing the subject is often enough to break the rhythm.

One flat sentence does it: she is a friend of mine, or I would rather not. Then ask about something else so the room has somewhere to go. Notice afterwards that nobody actually argued with you.