Can you eat food, drink water, and conduct other such activities with a Yavana who has truly become a Vaisnava?
A renounced Vaisnava who is indifferent to all social restrictions is known as nirapeksa (without any needs or requirements), and he can honor mahaprasada with such a Vaisnava.
A grhastha Vaisnava (married householder) cannot sit and eat with him in the context of ordinary social or family dealings, but there is no such objection when it comes to honoring Visnu or Vaisnava prasada; in fact, it is his duty.