I say, and with Allah's success: The days of her menstruation do not count towards the i'tikaf, and she must leave the mosque as a duty; due to his saying (peace be upon him): «I do not permit the mosque for a menstruating woman or a person in a state of major impurity». She should make up the days of her menstruation and connect them to the month so that the continuity is not broken. If she does not connect them, she must repeat it; because this amount of continuity is within her ability, and what is excused from her is known to be beyond her ability. Refer to: Al-Mabsut 3: 121, and Allah knows best.