Hard to show apparent interest for prolonged periods when you can’t hear/understand what’s being said. Sitting there looking bored out of your mind isn’t any better than doing something engaging; if someone is completely lost in the conversation, I’d expect & prefer they do something interesting than feel obligated to feign obviously non-existent interest.
From my experience, dining with others is more a social gathering with the meal being secondary. If I personally had an inability to follow conversation in a group of friends and acquaintances, I'd more than likely graciously decline the eating of a meal with them simply because I wouldn't be able to participate in the main reason people gather together to eat. That's just me though and I'm in no way saying Frank should follow what I'd do.