Answer
Wa `alaykum As-Salamu waRahmatullahi 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:
As long as the drink is lawful, there is nothing wrong to take a sip from a glass used by a non-Muslim friend.
In responding to your question, Dr. Muhammad Salama, PhD, Islamic Studies in English, Associate Professor at the Faculty of Islamic Sciences al-Madinah International University (Mediu), states:
As long as the drink is lawful, there is nothing wrong to take a sip from a glass used by a non-Muslim friend. The problem, however, could be with unnecessary intermingling with female colleagues which opens the door for sinful acts.
Islamic teachings have laid restrictions on the interaction between marriageable men and women to block all ways to evil practices.
Islam takes prohibitive measures and does not wait for evil to take place. Unnecessary mingling between marriageable men and women is not permissible and any contact has to be within the Islamic guidelines in this regard.
All evil practices between men and women are the results of neglecting such teachings. Many non-Muslims have converted to Islam when they realized these facts and compared them with the loose life style they live in with all its negative impact on individuals, families and the whole society.
Allah Almighty knows best.