According to the commentary, she is eating a Big MAC and an UMEboshi-&-tuna onigiri.

Ootori Gen(Leo)'s friend's sister, UMEda Kaoru was killed by Silver Bloome in Episode 40. She was played by Tominaga Yoshiko, also known as "Tominaga Miina," who became the third voice actress for Sazae-san's Isono Katsuo in 1998.
(“Katsuo” is a Japanese name for Skipjack tuna.)