The most famous has to be Eric Clapton’s ‘Layla’, when he was in love with Patti Harrison, wife of George, his close friend and Beatle. This is an unplugged version that is ok but rather misses the pain of the original, showing how the pain has lessened with time.
This is a similar scenario, Bobby Womack’s ‘I Wish he didn’t trust me so much’ supposed to be about falling in love with the wife of his friend Sam Cook:
I’ve just been watching the brilliant David Duchovney series Californication, which ic packed with celebs, including Rick Springfield famous for Jessie’s Girl which fits on this blog.
Here’s a really old one, from Lookin Glass, ‘Brandi’ It’s a traditional song about a girl falling for a sailor, but for a change he doesn’t promise to return, and she lives with it.