'Umrah is not fardh on her in this case; so, she does not need to worry about it. She can still gain similar rewards by doing other virtuous deeds that she can afford to As the Prophet (peace be upon him) told his beloved wife, Aishah, "Menses is something that Allah has decreed on the daughters of Adam." And, as Imam Shah Waliullah said, Islam always provides alternatives for those who are not able to do a particular act of virtue because of reasons beyond their control, and thus through them, they can be blessed with the same rewards.
In the case of hajj, however, she can do the tawaf while menstruating-if she is left with no other choice.
For further details on this, you may consult the following answer.