A simple yet effective conflict could be jealousy. For instance, if one friend gets a great job opportunity while the other is still struggling. The less fortunate friend might start to feel jealous, which could strain their relationship.
Betrayal is a strong source of conflict in a friendship story. If one friend reveals a secret that the other had trusted them with, it can shatter the bond of trust between them. The betrayed friend would feel hurt and angry, and the relationship would be in jeopardy.