Like Bill said, "man up" can be seen as as a stupid phrase. However, it does mean to stop feeling sorry for yourself and be an adult (male) instead of a child or baby. So, "woman up" would mean the same thing, addressed to a woman: don't be a child, be an adult (woman).
Depending on tone, it could be a rebuke, or it could be friendly and helpful like you said--like "cheer up."