It would be polite for you both to attend the lunch and worth the effort for the brownie points in future, she might even enjoy it. I was 15 years old when I met Mr P, his family were very sociable, mine were quite introverted so I struggled with such things for many years. We moved away after marriage and I gained confidence once I realised that lots of people felt like me, I started making an effort to look for those who looked uncomfortable and chatting to them. It's a bit like taking part in a play, act out the role of a confident, friendly person and eventually you will become so. I will never be the life and soul of the party but nobody intimidates any more. Suggest this role playing to your daughter, it may help her.