He was going to masturbate but his sister deceived him into thinking he'll die if he does. I'm not sure if such idiocy on his part is possible but lot's of stupid things are believed about women.
It's a good thing she didn't gain weight otherwise she should have kept starving herself. Being overweight is absolutely unforgivable for which starving to death is a small penance.
Thank god Tarumi didn't get invited along. It's annoying bullshit when a plan is made and one of the parties brings uninvited guests. The only time I liked this is in a manga where the girl is intentionally being an asshole and is painted as such.