Wa alaykum salam wa rahmatuLlahi wa barakatuHu,

Intercourse with a menstruating woman is unlawful. And only after the menstruation stops and she purifies is it again lawful. Ibn Hajar mentioned in his commentary on BaFadl,

ويستمر تحريم ذلك عليهما إلى أن ينقطع وتغتسل أو تتيمم بشرطه

“And the unlawfulness of that continues until it stops and she performs ghusl, or makes tayammum with its condition.” (v. 1, p. 198)

Sh. Sulayman al-Kurdi commented on the words “with its condition” to mean in the absence of water. (al-Hawashy al-Madaniyyah v. 1, p. 198) Meaning, she can make tayammum when water is absent.

And Allah knows best.

Shafiifiqh.com Fatwa Dept.