Sentence

Can someone check if this sentence below is correct?

For a bit of context, John has a girlfriend who gets extremely jealous when he talks to other women. So, when she found out that John was talking to one of his female friends, she asked him to unfriend her. John was reluctant at first, but he still did it.

After a few days, John's friend found out about it. She got mad and demanded John to state his reasons for unfriending her, but John didn't utter a single word since he didn't want to tell his friend that his girlfriend made him do it. John then realized that what he did is plainly wrong and told his girlfriend about it. Thankfully, John's girlfriend understood the situation she has put John in and decided to apologize to John's friend and tell her the truth. However, John's friend didn't reply so she just decided to message one of their mutual friends a message that reads " please tell her I'm sorry if that cause her pain to not know why John unfriended her."

Now, my question is: is this correct? "please tell her I'm sorry if that cause her pain to not know why John unfriended her."

I'm not entirely sure if it's correct. That's why I'm asking. If it's not, how would a native speaker phrase it?