I had to add an extra option number 5
.
My first instinct was that, while yes they were on a break, it's total BS that it would then be OK for him to straight away go off banging some random girl before knowing for certain what was going on with them. But then I remembered that he called her and heard Mark at her apartment... and although he got it wrong... at the time HE fully believed that she was taking the "break" thing as an opportunity to have sex with Mark... so his mistake is maybe justifiable from his perspective, if not overall...
... at the time that it happened, he really believed that he was only doing the same thing that she was.
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