Wa `alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
In this fatwa:
1- It is great of your friend to be caring for her parents, which is the duty of every Muslim, male or female.
2- She should not neglect marriage as it is a vital requirement for humans and also a Sunnah of the Prophet (peace and blessings be upon him).
3- She can do both; she can marry and take care 0f her parents at the same time.
Responding to you question, Dr. Muhammad Qatanani, Professor at the Islamic American University and Imam of the Islamic Centre of Passaic County in New Jersey, stated:
Every Muslim is aware of the virtues of taking care of his parents. At the same time, it is highly recommended to get married.
Allah Almighty said, “Your Lord had decreed, that you worship none save Him, and (that you show) kindness to parents. If one of them or both of them attain old age with you, say not “Fie” unto them nor repulse them, but speak unto them a gracious word. And lower unto them the wing of submission through mercy, and say: My Lord! Have mercy on them both as they did care for me when I was little.” (Al-Israa’ 17:23-24)
Also, the Prophet (peace and blessings be upon him) said, “O you young men! Whoever is able to marry should marry, for that will help him to lower his gaze and guard his modesty.” (Al-Bukhari)
The importance of the institution of marriage receives its greatest emphasis from the following Hadith in which the Prophet (PBUH) says, “Marriage is my Sunnah. Whosoever keeps away from it is not from me.” (Ibn Majah) Hence marriage is Sunnah for both men and women.
So, my advice for your friend is to combine between the two good actions; to take care of her parents and to get married. Doing this, she should try to find somebody who would help her to achieve that purpose.
Allah Almighty knows best.