Sitting alone whilst your lovely, kind, nice all round friend who everyone loves, flits about talking to others for most of the time on an evening out, doesn’t sound like a barrel of laughs to me. So much better to stay home and not feel awkward or self conscious. Looks as if this ‘so lovely’ social butterfly just wants a “wingman” - or the female equivalent, to make herself feel comfortable and in case the evening turns out to be unexpectedly quiet!
Not a lot of communication here and it looks as if the often abandoned one hasn’t had the courage to put a stop to this behaviour. Think her friend is being very selfish or, at best, thoughtless and rude. If she’s so lovely, she’s not going to mind the truth….they both expect different things from their night out and the often abandoned one would clearly prefer to spend time in a one-to-one outing or a small fixed group. Any awkward refusals or excuses would be more likely to spoil this friendship than speaking the truth, I should think. So, no need for unpleasantness -but don’t feel used - if you both want different things, then say so and, as friends, learn to listen and respect that….or maybe don’t be surprised when your rather odd evening out invitation is declined.