I had a similar experience, so I will help you. When my friend asked me to do it, I didn’t know how to do it right. I was also worried about how to balance the tone of the letter. While searching for information, I came across an article about a reference letter for a friend for a job at https://jobssite.ca/advice-on-writing-personal-reference-letter-from-a-friend/ . Here I found out how important it is to find the golden mean between formal and informal style. The tips from this article helped me a lot, and I was able to find the right style for a letter that sounded natural but still remained professional. I definitely recommend reading it!