"Writing to a friend" in Engpsh can be expressed as "writing to a friend" or "composing a letter to a friend." The phrase "writing to a friend" is a more general term that can refer to any form of written communication with a friend, such as an email, text message, or social media message. "Composing a letter to a friend" specifically refers to the act of writing a traditional letter, which is a physical piece of paper with writing on it that is sent through the mail.

Here are some examples of sentences using these phrases:

"I spent the afternoon writing to my friend about our recent vacation." (我整个下午都在给我的朋友写信讲述我们最近的假期。)
"I always enjoy composing letters to my friends. There's something special about receiving a handwritten letter in the mail." (我总是很喜欢给朋友写信。收到邮寄的手写信件有一种特别的感觉。)