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First of all, your question is about "what to write" rather than "how to write". Now (before your question can be put on hold), take a good look at your characters. What conflict of interests coul...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/29890 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
First of all, your question is about "what to write" rather than "how to write". Now (before your question can be put on hold), take a good look at your characters. What conflict of interests could develop between them? If no obvious conflicts are seen, maybe there is a scenario (a love interest, a lottery win etc.) that can cause this conflict? Next, see that this conflict would develop as a betrayal rather than an open argument. Betraying party should have some reasons to pursue his/her interests in secret from the other party. Last, make this betrayal revealed, and either have it too heinous for the friend to forgive, or have an argument between friends develop in a way that ends up in a deeper division rather than truce.