I was in a similar situation when I was in 8th grade. I was new in school and made a few friends, but when they were gone I would be alone. My school was also very cliquey and I didn't really fit in. Before she moves, maybe see if you can find another person sitting alone and maybe try to sit with them and make small talk. If by the time she moves and you're still stuck in this situation I would just eat, and then go to the bathroom and play on your phone if y'all can't have phones out during lunch period. That's not the best advice, but that's what I use to do sometimes and it made me not so anxious, plus I'd play games! Let us know if you end up finding a new friend. Wishing the best for you.