they aren't friends. they had to work together and saved each other's lives, and Negan saved little Hershels life.
They did a really good job with Negans character. he truly has changed, but still has the cocky personally. Plus, Daryl and Carol do get him to do the dirty work when it came to the whisperers. the last Episode, Negan truly tried to make amends, but Maggie wouldn't let it happen. she does accept that Negan has changed, but she basically refuses to accept
his apology.