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.
Points to Consider:
1- Islam cares about the dignity and honor of Muslims.
2- Islam forbids a man and a non-mahram woman from being in privacy away from people.
3- A man can lead a non-mahram woman in congregational prayer as long as they are not in an isolated place.
In his response to your question, Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states:
It is not wrong for the Muslim sister to pray behind you in a room unless you are secluding yourself with her. There is no prohibition in the Shari`ah so long as we do not isolate ourselves with members of the opposite sex.
Prayer is the most excellent deed that we can perform and we should try to pray in jama`ah as much as possible, for it is 27 times more rewarding than prayers done individually.
Thus, you ought to find a place where you can pray without being isolated with each other. But if there is no such place available and you would be forced to seclude yourself in a tight room, then you must pray separately.
Dr. Sano Koutoub Moustapha, Professor of jurisprudence and its principles at the International Islamic University, Malaysia, adds:
Islam does not prohibit a woman to pray with a non-mahram man at all as long as the prayer is taking place in an open area. Accordingly, there is no harm for you to allow your secretary to join you in prayers if she wishes so.
Almighty Allah knows best.