Moral dilemmas and temptations, relationship friction, duty and responsibility, losing an advantage

Discovering a secret from a friend's past that colors the character's view of them, such as his role as a bully, abusive behavior toward animals, or reckless behavior that endangered others
Learning a friend is cheating on their partner—perhaps with the character's own spouse
Finding out the friend holds extreme political or social views, such as racism or sexism
Learning the friend was responsible for a major blunder that impacted others—a blunder they denied playing a part in
Discovering a friend is secretly competing with the character for a job, a romantic partner, a prize, etc.
The friend being involved in something that runs counter to the character's values, such as embezzling funds or harassing others 
Overhearing the friend speaking badly about the character
Learning the friend has shared the character's secrets or struggles with others

Having to confront the friend
The friendship becoming awkward because the character is pulling back
Not knowing what to do and having to act in the meantime like nothing is wrong
Telling someone else about the situation and them taking the friend's side
Frustration with the friend for their underhanded dealings
Learning that others knew about the issue and didn't tell the character
Conflict with other friends who feel they have to take sides and can't stay neutral
The character's reputation being questioned because of their association with the friend