I believe that they went to elementary school together. Remember when Mo was being accused of being a thief by his school mates as a child? I think She Li "saved" him back then. Although more likely She Li framed him, spread the rumors, and then "saved" Mo.
I suspect for years She Li has been manipulating Mo and keeping him under his thumb. His plan earlier was to get Mo expelled and have Mo work at his father's bar where he'd be She Li's little thug. However, He Tian freed Mo and keeps She Li at bay. She Li probably thought He Tian would get bored of Mo, but now he's realizing that He Tian really likes Mo and it's ticking him off.
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Wow that's a very precise theory.(๑•ㅂ•)و✧
I wonder what She Li did for Mo and Mo''s mom was a hot little number lol