IDIOMS

drift apart

(idiomatic, intransitive, of companions) To lose contact, to lose one's friendship or closeness.

Example:   We used to be good friends but we dripted apart over the years.‎
1931, John Klenner, Just Friends
  To think of what we've been, and not to kiss again, seems like pretending it isn't ending, two friends drifting apart, two friends but one broken heart