The following page of the FF7 Remake guide contains a walkthrough for Lost Friends side quest available in Chapter 3 - Home Sweet Slum. Your objective is to find three lost cats in the Slum.
Talk to Betty, and you will find out her cats are missing. The girl will ask for your help to find them. The first cat is outside the bar where you e.g. met with Tifa and the rest of the team.
The other cat is in the street, not far from the road leading to the train station.
The third cat is harder to find. You have to go under the tin tunnel to reach it.
Go back to the girl and she will thank you for your help.
Reward: A Maiden's Kiss (removes curse and frog status).