Even though Islam is not a form of idolatry it is an Emuna Zara and one who follows them is an Apikores, a real one. Getting up in front of a religious group to say their religious chant is an obvious admission to their religion. All the famous arguments of Mechazek yedei Kofrim, and all that, apply here.
How weird it is, that those who easilly yell, Cuffrim, at many occasions will stick up for actual Kfira.