If the clothing is something particular for men, then it is forbidden for her to wear it regardless of whether she is praying or is outside of the prayer. This is because it is confirmed from the Prophet (sallallaahu alaihi-wasallam) that he cursed those women who imitate men and appear like men as well as those men who imitate and appear like women. It is not allowed for a women to wear clothing that are particular for men and it is not allowed for men to wear clothing that are particular for women.
However, we must understand the concept of "particularity". Particularity is not with respect to colour but it is with respect to colour and description. It is, then, permissible for women to wear white clothing as long as it is not the same as the white clothing of men. If it is clear, though, that women wearing men's clothing is forbidden, then her prayer in such clothing is not valid acording to those scholars who say that the covering in the prayer must be covering which is permissible.
This is a question in which there is a difference of opininon among the scholars. Some scholars say that it is a condition that the covering or the clothing that is in itself permissible. Some do not lay down such a condition. The proof for those who lay down such a condition is that the covering of the aurah [what must be covered] in the prayer is one of the conditions for the prayer and the covering must be something that Allah has permitted. If it is something that Allah has not permitted, then it is not considered a legal covering since it goes against what is commanded. The proof of those who say that the prayer is still valid while the person is committing a sin is that the women has actually covered herself and that the sinful aspect is something that is external and is not particularly related to the prayer. in any case, the person who prays in forbidden clothing is in a dangerous situation since her prayer may be rejected and not accepted from her.