The expression is en venir à. When used in passé composé, you only have to conjugate the auxiliary verb être and to adjust venu with the correct agreement (gender and number).

It is not necessarily pejorative as in to be reduced to (something). It can rather be translated by to end up (doing something). It is only as pejorative as the action behind it.

For example (with a plural so you see how it goes) :

Leur voiture étant cassée, ils en sont venus à la location.
(Their car being broken, they ended up renting one.)

I would not take “en venir à” for a real gallicism. “venir” could be replaced by “arriver” (with a slight hue of some effort), or “parvenir” (with definite exertion) – and the reverse “déchoir”, rather in this case “déchoir au point de” ; and “en” means nothing more than “grâce à” or “à cause de”.

It could be used at any time and mood of the verb.
“Il est très apprécié comme chef de rayon, il en parviendra à être gérant”.
“S’il n’avait été soigné à temps, il en serait aujourd’hui déchu à l’alcoolisme incurable”.

Pitfall : “un parvenu” = a parvenu, an upstart.