In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
It is not necessary to wash disposable sanitary pads before throwing them away.
In responding to your question, Sheikh M. S. Al-Munajjid, a prominent Saudi Muslim lecturer and author states:
Washing Sanitary Pads: Necessary?
None of the trustworthy scholars have stated that a woman has to wash her sanitary pads or rags which may have traces of menstrual blood if she is going to throw them away and not use them again.
The female companions of the Prophet Muhammad (peace be upon him) did not wash these sanitary pads. Abu Sa`eed al-Khudri said; “It was said, ‘O Messenger of Allah, should I do wudu from the well of Buda`ah which was a well into which were thrown menstrual rags, the flesh of dogs and other putrid things?
The Messenger of Allah (PBUH) said; “Water is pure and nothing makes it impure.” (At-Tirmidhi)
This is an authentic hadith which was classed as such by Ahmad, Yahya ibn Ma`een, Ibn Khuzaymah, Ibn Taymiyah and others.
What is referred to here by “menstrual rags” is the rags which were used for menstrual blood, as al-Mubarakfuri said in his commentary on this hadith.
So, it seems that they used to throw them away when they were contaminated with blood; otherwise the Sahabah would not have raised the issue of whether the water into which these rags were thrown was pure or not.
Myth about Unwashed Sanitary Pads
There is no sound basis for what your friends have said; rather it is a myth, which has no basis. Such notions are widespread among the common folk. So you have to ask for proof and evidence, and refer to the scholars and books of knowledge.
You have to reject these myths and warn people against them. Each person must protect himself against the evils of the devils among mankind and the jinn by remembering Allah a great deal (dhikr), reciting Quran and adhering steadfastly to the commands of Shariah, and keeping away from myths and stories that have no proof.
Almighty Allah knows best.