All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
Every child is a trust in the care of their parents – they are responsible for them before Allaah. Ibn ‘Umar said: I heard the Prophet say: “Every one of you is a guardian and is responsible for those under his guardianship … and a man is a guardian in his house and is responsible for those under his guardianship, and a woman is a guardian in her husband's home and she is responsible for those under her guardianship.” [Al-Bukhari and Muslim]
Therefore, it is not permissible to let the grandmother perform chants on the child in accord with her rituals of unbelief. In addition to being forbidden, this could lead to the child being affected by his grandmother, and could even be a cause of him being led astray in the future.
So, the wife should advise her husband in a gentle and soft manner and explain to him the seriousness of this matter, and that it is not permissible to be negligent about it. She also has the right to demand a separate house where she can avoid anything that could cause her distress. In Islam, she is not obliged to live in the same house as her mother-in-law. Please refer to Fatwa 84608.
What the mother-in-law said – that she can use whatever means she likes to heal the child – is not correct. Her son, i.e. the husband, should explain to her that this is something that our Sharee'ah does not agree with, and it is not permissible for him to agree with her on matters that involve falsehood. However, he should be gentle with her and if she gets angry, he should be dutiful to her until she is satisfied. For more benefit, please refer to Fatwa 275419.
Allaah Knows best.