Mom is not going to accept Sensei's conditions for three reasons: The cover page of the chapter says she will not let go of her son. When Shiba tells Mom what to say to Reiji, mom's response was "you do understand". When talking to the nurse about children, mom said they "make your whole world". She will not let Reiji go under any circumstance and that means Shiba's terms have fallen through.
Reason she called Gen is, is most likely because she knew Reiji could have only found about her illicit relationship with Gen's dad from Gen himself. She probably wants to know why he told Reiji and perhaps something more. We can gleam three potentially important details from this:
-Gen came down to that house at a moment's notice. This means Mom has 'some' influence over him, including his contact information.
-Mom may find out about Nagi
-Gen may find out about sensei
From the author's twitter:
"果たして柴ちゃんは勝ったのか…
あるいは眠れる獅子を起こしてしまったのか…"
" Did Shiba win?
Or did you wake up a sleeping lion ..."
All implies sensei just made a huge mistake.
