You are right, the masculine version of that would indeed be "si è dimenticato", while the feminine version would be "si è dimenticata".
In this specific case, the verb is actually "dimenticarsi" rather than "dimenticare".
"Dimenticarsi" is a reflexive verb (here is more info about reflexive
verbs: https://members.rocketlanguages.com/members/italian/lessons/466) while "dimenticare" is not.
Depending on the form you wish to use, whether the reflexive "dimenticarsi" or the regular "dimenticare", you need to adjust the ending accordingly.
In the reflexive verb, you will need to adapt it to the subject, as in:
Lei si è dimenticata
Loro si sono dimenticati
...and so on.
In the second case (regular verb), then ending wouldn't change, as in:
Maria ha dimenticato la poesia.
(Maria forgot the poem.)
Carlo ha dimenticato come si dice "grazie".
(Carlo forgot how to say "thank you".)
Hope this helps!