If her usual place of residence is in her state, and she is only visiting her husband for a few days, then her husband's house is not regarded as her home; rather it is like any other place to which she may travel, if the distance between the two states is the distance at which shortening prayers becomes permissible, which is approximately 80 km. So she should shorten her prayers during this trip, but if she has decided to stay with her husband for more than four days, she should offer the prayers in full from the time she enters his state, because in that case she comes under the same ruling as one who is a resident. But if she intends to stay for four days or less, then she should shorten her prayers.
When it is permissible to shorten the prayers it is also permissible to put prayers together. It is also permissible to put prayers together for other reasons, such as illness or difficulty (in offering each prayer at its time).
But there is a difference between shortening prayers and putting prayers together. Shortening prayers is a confirmed Sunnah (Sunnah mu'akkadah) for travellers that should not be neglected. As for putting prayers together, it is permissible, so the traveller may put prayers together or he may offer each prayer at its particular time.
Shaykh Ibn Uthaymeen (may Allah have mercy on him) was asked: I live in al-Qaseem, but sometimes I go to Riyadh to visit my family; I have a room there and I stay with them for two or three days. Can I shorten my prayers if I pray on my own? Do I come under the ruling on travellers?
He replied: Yes, you come under the ruling on travellers, because your place of residence is al-Qaseem, and when you visit your family it is the visit of a traveller. Hence the Prophet (blessings and peace of Allah be upon him) shortened his prayer in Makkah, although he had previously been a resident of that city and he had houses there. But when he migrated, Madinah became his place of residence. So if you go to visit your family, you are a traveller.
End quote from Liqa' al-Baab al-Maftooh, 58/23
He was also asked: If a person travels and leaves his family, and he has the intention of not returning to his homeland, and he stays in a city that is far away from their city, with the intention of settling there, then he goes back to his family for a visit, should he regard himself as a traveller or not?
He replied: If a person leaves his original home with the intention of settling in a different city, then when he goes back to his original home he is a traveller, so long as he still has his original intention (of settling in the second city). The evidence for that is that the original home of the Messenger (blessings and peace of Allah be upon him) was Makkah, but when he conquered Makkah he shortened his prayers, and during the Farewell Pilgrimage he shortened his prayers.
Questioner: Does this apply even if his wife and children are in the first city?
Shaykh: Yes, even if his wife and children are there.
End quote from Liqa' al-Baab al-Maftooh, 82/23
For more information please see the answer to question no. 121637
And Allah knows best.
Reproduced from Islam QA