Exactly! He did rape Tsubaki because Tsubaki was underage while the boyfriend was most likely too much older than he was. BUT I agree that it is highly unlikely he ever told his mother the extent of their relationship. He already felt guilty enough as it is. Which is part of the reason why I find it so unbelievable that people are blaming Tsubaki for the relationship because they don't think he felt guilty enough, or some such B.S. like that. On top of the fact that it makes it even harder to see that when numerous OTHER posters are finding it so unacceptable that his mother is so calm about their relationship. I mean it never ONCE occurred to them that the REASON she was so accepting was precisely BECAUSE her son felt too guilty to tell her everything? I find that REALLY weird.
I concede that you are most likely right, he probably didn't tell her everything. From the beginning the BF was also much younger than the mom, so it could be reasonable to think of the kid being 16 (although that would be generous looking at him in the story) and the bf being around 26, which isn't that huge of an age gap. However, one is still an adult and one still a child. I could see the mom saying something like "You are obviously happy, so I'll support you" but also laying down some ground rules about no sex or living together until he graduates. Other circumstances I would say she should be angry as he was the adult in charge at the time, but I think that was more left to the staff and he was just there as a guest.
I see a ton of comments about cool mom, but I'm not seeing it the same. Supporting your child in their relationship and happiness is cool, yes. Finding out your boyfriend raped your child and helping them continue the relationship, not cool. He is still an underage kid