Man that girl is annoying. Next you're gonna tell me she's lesbian and she has hots for Toiro.
As whether MC has to go defend his friend, where is the danger in the first place? Dude just confessed, got turned down, he persist a little (persisting is not a crime or inherently bad) before whatshername teleported to slap his hand away from Toiro. If you consider touching to be a crime itself, since the muscle dude was trying to make Toiro face him by physically grabbing her no doubt, then I expect the same consideration when Toiro's friend grabbed on Masaichi and pulled him back, or when she slapped the guy's hand. At the point where muscle guy still tried to appeal, it wasn't still a cause for concern (harrassement), it's what happen next IMO. If he continued to be physical or not leaving her alone then yeah, as childhood friend Masaichi should go intervene.
Still, I laughed when Toiro joked about the muscle guy and Masaichi joined in, it looks to me as her trying to cheer up her childhood friend's bruised ego. Masaichi, if you really feel down because you were jealous of the guy's physique, don't just lament of what you don't have and start training up. Put less time playing and more time working on something you feel lacking. Something I find most often lacking in romcom like these, is where are the parents? Is it me or often in these mangas the parents aren't here to notice if their child is down, or doing some parenting and encourage their child to do have good habits.