If a Jew converts to Islam should he still observe the Sabbath?

I have been told by my Muslim connections that there is a story in the Quran that relates how wicked the Jews are because they would lay traps to trap fish on the Sabbath. My question is given that the Quran obviously condemns Sabbath desecration by a Jew would it not make sense that a Jew who converts to Islam is still obligated to observe the Sabbath being under the covenant of the Torah which the Quran seems to accept was, at least in its original form, of divine origin? To my horror when I asked this question to my Yemenite Arabic teacher he answered that a Jew that converts should not continue to observe the Sabbath. Surely he is mistaken? Did Islam come to change the Torah? Surely that cannot be the official position of Islamic scholarship?