It is better to declare the faith, as it is important in many social matters. For example, in the event of death, if faith is not declared then Muslims will not be allowing funeral in Muslim grave yard. Also, this means she will have to hide any sign of being a Muslim and it can cause unnecessary hardships and may sometimes require her to stop praying or fasting just to avoid being seen by others.
But if there is real reason to hide faith like fear of being harmed by others, then this can be done. In Quran there is an instance of a person in the times of Musa(a.s) where he is mentioned as a person who concealed his belief.
And a believing man from the family of Pharaoh who concealed his faith
said, "Do you kill a man [merely] because he says, 'My Lord is Allah '
while he has brought you clear proofs from your Lord? And if he should
be lying, then upon him is [the consequence of] his lie; but if he
should be truthful, there will strike you some of what he promises
you. Indeed, Allah does not guide one who is a transgressor and a
liar. Quran 40:28