YOU SAID:

I went to a sleepover and my friend became a zombie.

INTO JAPANESE

私はお泊まり会に行きました、そして、私の友人はゾンビになりました。

BACK INTO ENGLISH

I went to a staying party and my friend became a zombie.

INTO JAPANESE

泊まりに行ったら友達がゾンビになりました。

BACK INTO ENGLISH

When I went to stay, my friend became a zombie.

INTO JAPANESE

私が泊まりに行ったとき、私の友人はゾンビになりました。

BACK INTO ENGLISH

When I went to stay, my friend became a zombie.