I ate so much I thought I would eat all of the restaurant's food.
"店の食べ物すべて食べつくすかもと思ったぐらい沢山食べてた"
- They ate so much I thought they would eat all of the restaurant's food.
- They ate so much that I thought they might eat all the food in the restaurant.
They -> he/she
I went to an all you can eat with (friend's name) and they ate so much I thought they would eat all of the food in the restaurant.
- (友達の名前)と食べ放題に行ったら、店の食べ物すべて食べつくすかもと思ったぐらい沢山食べてた
My friend ate so much I thought he/she was capable of eating all of the restaurant's food.
ご質問ありがとうございます。
この場合、友達について、「店の食べ物すべて食べつくすかもと思ったぐらい[沢山食べてた](https://eikaiwa.dmm.com/uknow/questions/72524/)」を言いたいなら英語「My friend ate so much I thought he/she was capable of eating all of the restaurant's food.」と言えます。
ご参考になれば幸いです。