A woman had her menses but forgot and prayed; what must she do?

Prayer is neither obligatory for a menstruating woman, nor is it valid from her, and she is not sinful for neglecting it, due to the hadith of Aisha, may Allah be pleased with her. If she prays intentionally, she incurs sin, but if she prays out of forgetfulness, there is no sin upon her, due to the hadith: "Indeed, Allah has forgiven my nation for their mistakes, their forgetfulness, and what they are forced to do."