She prayed when she was junub by mistake – does she have to repeat the prayers?

Firstly: 

Whoever prays when he is not in a state of tahaarah (purity)
has to purify himself and repeat the prayer, according to scholarly
consensus, even if he forgot. Al-Nawawi said in al-Majmoo’ (2/78): 

The Muslims are
unanimously agreed that it is haraam to pray when one is impure, and they
are unanimously agreed that the prayer is not valid whether he knew that he
was impure or was unaware of it or forgot. But if he prayed not knowing it
or having forgotten it, there is no sin on him; if he knew that he was
impure and that it is haraam to pray when one is impure, then he has
committed a grave sin. 

Secondly: 

Ghusl following janaabah is not valid unless one has the
intention of doing ghusl, because the Prophet (peace and blessings of
Allaah be upon him) said: “Actions are but by intentions.” Narrated by
al-Bukhaari, 1; Muslim, 1907. 

The site of the intention is the heart; it is not required to
speak it out loud. 

If you remembered that you were in a state of janaabah before
you took a bath at Maghrib time, and you took a bath for that reason and
repeated the prayers that you had offered when you were junub, then that
ghusl is valid because the intention was there. You did well to repeat the
prayers as this is what is required of you. 

But if you did not remember that you were in a state of
janaabah until after you had had the bath, and you took a bath in order to
clean yourself or to cool yourself down, for example, then this bath did not
take away the janaabah because there was no intention to do so. In that case
you have to repeat the ghusl and the prayers. You repeated the ghusl, so all
that remains for you to do is to repeat the prayers. So repeat Fajr, Zuhr,
‘Asr and Maghrib. 

And Allaah knows best.