This page of the guide to Final Fantasy 7 Remake has a walkthrough for the Tomboy Bandit side mission available in Chapter 14 - In Search of Hope.
You start it by talking to Johnny. He will ask you to get his wallet back. It isn't a difficult task. You can do it during the Missing Children quest or Corneo's Secret Stash.
After talking to Johnny, go to church. On the spot, you will have a heated argument with Kyrie. She will send you back to the arena.
Go to the Colosseum and defeat Hellhound and Beastmaster. These enemies aren't demanding, use fire to defeat them fast.
After defeating the opponents, return to the church. Kyrie and her grandmother are waiting for you. This part of the mission will start a Discovery - we described it on a separate page.
To complete the mission, you have to go back to Johnny.
Reward: Corneo Vault Key, Johnny's Wallet.