Dizque

Su dizque amigo = his so-called friend (not a true friend, in my view).

Dizque is a useful word that mixes doubt with derision. Here are some examples:

Sus dizque amigos. His so-called friends. You doubt they are actually true friends of his, even if he thinks they are. Note that the form is invariable.
Ella dizque canta. Supposedly she knows how to sing but I've heard her and I'm not impressed.
Los dizque expertos dizque independientes. The supposedly independent faux-expertos. (Op-Ed example)

More formally, in these examples you could use supuestos and supuestamente, respectively. Según would work as well: Según canta. Supposedly she sings.

From these examples, you can see that dizque can modify both nouns, adjectives and verbs: dizque expertos, dizque independientes, dizque canta. It's both an adjective and an adverb.

I've seen dizque spelled with <s> instead of <z>. Since it is informal and more commonly encountered in speech rather than writing, some confusion over its spelling is expected. Apparently it is a contraction of dice que.

The more neutral word llamado can also be used to cast aspersions: Este llamado jefe de gobierno, this so-called jefe de gobierno, the highest-ranking local government official in Mexico City. Alleged, as used in police reports, would be presunto. El presunto asaltante, the alleged assailant.