I've seen this idea before but I don't really understand why. Rusty was an entitled douche when he and Jill first met, and they went on a single date where he couldn't be himself when his friends showed up. Then look at Rusty and Micha, he was fucking around like he always does and unexpectedly caught feelings. Then the tables were turned on him.
What does it really matter if MC is dating a girl that Rusty only had a brief history with? A misplaced sense of jealousy? Does he just now suddenly realize Jill is the love of his life? Or is she just "the one that got away" so any feelings he may have had are now heightened because she's seeing MC.