How can I stop messages from going to group chats on iPhone?

IWhen I send a message to my business partner/friend (who has an Android), it appears on my end as if I’ve sent it directly to him in a one-on-one conversation. However, on his end, the message shows up in a group chat that includes another employee of ours.

What’s more confusing is that the third person (the employee) is able to see and respond to the message, even though I never intended to include them—and on my device, it still appears as a private conversation with just my business partner.

Could you help me understand why this is happening and how to fix it? It has happened more than once and is causing some issues business wise.

[Re-Titled by Moderator]

Original Title: Tax for one recipient is appearing in a group chat