I understand that you're having trouble with group messages. It seems that your friend is not receiving your messages when send to a group. Are any other members in the group unable to see your messages when sent to the group?
I recommend to take a look at the following article for help with messages, specifically this section:
If you’re in a group message and stop receiving messages, check if you left the conversation. Open Messages and tap the group message. If you see the message You left the conversation, then either you left the conversation or you were removed from the group message. You can rejoin a group message only if someone in the group adds you.
You can also start a new group message. Here’s how:
- Open Messages and tap .
- Enter the phone numbers or email addresses of your contacts.
- Write a new message, then tap to send.
If you have other issues with a group message, you might need to delete the conversation and start a new one. To delete a group message:
- Open Messages and find the conversation that you want to delete.
- Swipe left over the conversation.
- Tap Delete.
When you delete a message, you can't recover it. If you want to save parts of a message, take a screenshot. To save an attachment in a message, touch and hold the attachment, tap More, then tap Save.
If you can't send or receive messages on your iPhone, iPad, or iPod touch
You may also want to have your friend take a look through these steps. You both may need to delete the thread on your phones and start a new one.
Let me know how these steps work out for you.