So he is their ally today, narrating the past state in which the devil adorned their deeds for them. Or he is their ally in this world, so today is a term for the time of this world. And the meaning of 'he is their ally' is their companion, and how evil is the companion. Or it can be understood that 'so he is their ally today' narrates the future state, which is the state of them being punished in the fire, meaning he is their supporter today, and there is no supporter for them other than him, negating any supporter for them in the most emphatic ways. It is also possible that the pronoun refers to the polytheists of Quraysh, that he adorned the deeds of the disbelievers before them, so he is an ally to them and these, because they are among them. It is also possible that it is understood by omitting the addition, meaning: so he is the ally of their likes today.