Boyfriend learned about "It's Complicated" through the bonus week of Friday Night Funkin' Soft
Considering that Friday Night Funkin' Soft takes place in an alternate universe, it would make nearly no sense for Boyfriend to even know about "It's Complicated" unless Benjamin, Grace, or Soft Pico told him about it beforehand. Lo and behold, the secret week has Benjamin and Boyfriend face off against each other. It's their only known interaction, even if it's in a dream, and considering that both "Beathoven" and "Glitcher" are based off of some of Boyfriend's earlier opponents, it's likely that Benjamin told Boyfriend about his own experiences, leading to Sayori singing this song during the Festival Week. In addition to the above WMG, Boyfriend has met someone from Friday Night Fever
It doesn't matter if it's Fever, Taki, or Tea, it just has to be someone who would believably know about "Crucify" and could believably teach Boyfriend about it.