Praise be to Allah.
So long as the clothes she is wearing do not contain any najaasah (impure substance), she is permitted to pray in them, even if she was wearing them during her monthly period. The menstruating woman herself is not naajis, neither is her sweat. The Jews used to think this, but Allah stated that this was a lie, as is reported in the saheeh hadeeth narrated by Muslim (may Allah have mercy on him) from Anas:
The Prophet (Peace & Blessings of Allah be upon Him) said: ‘Do everything except have intercourse.’ Word of this reached the Jews and they said: ‘What does this man mean, in every issue he differs from us!’" (Saheeh Muslim, 455).
If the clothes get some menstrual blood on them, it is enough to remove the impurity with water and then she can pray in them.
And Allah knows best.