This is what I have gathered from multiple sources (sauces). Keltie saw some pictures on Ryan's phone of him and this family friend (who happened to be a girl), assumed he was cheating and then dumped him. And people thought it involved Brendon cuz, 당신 know, they kind of act gay together, but we all know its not rly like that. So yeah. Basically, Kelties a 암캐, 암 캐 and Ryan deserves way better than her.
He found out she cheated on him, but didn't do anything about it, just kept quiet, but then he cheated on her and she dumped him. After that he spilled the news about Keltie. Really, if they both cheated on each other, they were never really as in 사랑 as they thought.