Is it permissible for a woman suffering from rheumatism to go to communal women's baths for treatment, based on the doctor's recommendation?

We advise you to supplicate frequently, for Allah is the Healer, and to be diligent in performing ruqyah (definition) (Islamic exorcism/healing), for it is beneficial by Allah's permission.

Regarding public baths, if there are no Sharia (definition) violations such as the exposure of 'awrah (private parts), then there is no harm, while being mindful of the potential presence of invisible cameras. It is preferable to choose trustworthy places. However, if there is anything that violates the Sharia, such as the exposure of 'awrah, then it is not permissible to go there, based on the Prophet, peace and blessings be upon him, who said: "Whoever believes in Allah and the Last Day should not let his wife enter the public bath." And his saying: "Any woman who takes off her clothes in other than her husband's house has torn the veil between her and her Lord."

If a woman has a necessity or need, then there is no harm in her going, provided she remains modest and covered, and lowers her gaze from what is forbidden, as Ibn Taymiyyah permitted it when there is a need, along with lowering the gaze and protecting one's private parts. However, whenever possible, seeking healing away from these baths is preferable and safer.