Is it obligatory for a menstruating woman to perform istinja' even though she does not pray?

"Istinja' (ritual cleansing after answering the call of nature) is not obligatory except when intending to pray or purify oneself. There is an opinion that it becomes immediately obligatory if there is a concern about the impurity spreading or if water is unavailable. As for a menstruating woman, istinja' is not obligatory for her because she does not pray. However, being soiled with external impurity is forbidden."