The friend whose wife that the song was written about just happened to be
George Harrison,
lead guitarist for
The Beatles. As is said above, Eric eventually did
marry her (her actual name was
Pattie). The relationship between Patti and Eric caused George to get very upset, and greatly strained the friendship of the two (as one would expect).
Also, the song was originally performed by Derek and the Dominoes, one of Eric Clapton's many groups. It appeared on their only studio album, Layla and Other Assorted Love Songs.