What is required is to do ghusl if water is available and one is able to do that, because Allah says (interpretation of the meaning):
“And when they have purified themselves, then go in unto them as Allaah has ordained for you”
So she has to purify herself with water after her menses or after postpartum bleeding stops, then he may come to her. But if they are on a journey or she is sick and cannot use water, or water is not available, then tayammum takes the place of ghusl because it is a means of purification.
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him)