My wife is a Muslim. She wants to write a will. I know her non-Muslim relatives can't get part of her inheritance, but can she will some thing to her non-Muslim relatives in the up to 1/3 part (Wasiyeh) that she is allowed to will to whom ever she wants?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
It is lawful for the Muslim to make a will bequeathing his relatives who are non-Muslims provided that what he writes does not exceed the third of what he owns.
Ibn Al Qayyim said in his book "Rulings concerning Ahl Zimma: "The validity of the will made to a given Zimmi (non-Muslim who has a covenant to live among Muslims)".
Ishaq Ibn Mansoor and Abdullah Ibn Al Imam asked Imam Ahmad "Can the Muslim make a will for some non-Muslim relatives?" Imam Ahmad said: "Yes". Safiya (), one of the Prophet's wives, made a will for a Jewish relative of hers".
The story of Safiya's will was narrated by Said Ibn Mansoor in his Sunan. He said: "Sufiyan reported from Ayub from Ikrima that Safiya Bint Huyay sold her room to Muaviyah for one hundred thousand (Dirham). She had a Jewish brother. She called him to Islam, but he refused. Then she made a will for him bequeathing him a third of the hundred thousand (Dirham)".
Allah knows best.