She is loyal wife even after her divorce. She has sex with Masuo in the first place is because a girl tricked her into it. Later in order not to lose the only human connection she has, she let him sleep with her.
Hitomi Ito and Hitomi Onizuka (Mrs. Yamada) are both separate entity. She uses Hitomi Ito's and pretends to be a senior in Masuo's college to keep an eye out on him and live her college lover fantasy. She doesn't know Kunio that well.
On the other hand, the Hitomi Ito you are calling Kunio's girlfriend has yet to appear outside of that one cutscene.