All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
A woman is not permitted to give another woman a massage that necessitates uncovering the 'Awrah, because it is forbidden to unveil the 'Awrah without a necessity or need, such as treating an illness, and while there is no other way that is free from the prohibition, to treat the illness.
The Muslim woman's ‘Awrah with another Muslim woman is between the navel to the knee; this is the same thing in regard to her ‘Awrah in front of a non-Muslim woman according to the preponderant opinion of the scholars.
For more benefit, please refer to Fataawa 109808 and 85824.
Allah knows best.