No. No one who denies the Son has the Father. They may claim to worship a singular god all they like, it is not the Blessed Trinity, and nowhere in St. Pius X Catechism does it say they worship God.
Wouldn't it be fair to say they worship the True God insofar as they worship "The Omnipotent, Omniscient, Omnipresent Being who Created The World" and they worship a false god insofar as they worship "The God who sent us Mahomet and The Quran and is Definitely Not a Trinity"?
It could even be argued that it depends on the prayer, if a muslim says
"Oh I worship you Allah Eternal Emperor of the Universe, have mercy upon me"
he's referring to God as Being
but if he says
"Oh I worship you God of Mahomet who revealed to us the Quran"
he's referring to the specific demon that spoke to Mahomet.