What must she do after her husband fondled her from behind, and she is unsure whether penetration occurred, and she decided to fast for two months as expiation for that?

If a husband has intercourse with his wife during the daytime in Ramadan, whether anally or vaginally, his fast is broken, and he must make up the day (qada') and offer expiation (kaffarah). The major expiation becomes obligatory if the intercourse involved full penetration of the glans. If foreplay does not result in full penetration of the glans, then expiation is not required. If penetration occurs, the husband must offer expiation and make up the day for each day [this occurs], while the wife, according to the more preponderant opinion, only needs to make up the day.