To me, this is the million dollar question, and as best I can tell this far, the answer to it hinges on how one defines “formal.”
If formal simply mean intention, then it would seem to be remote material cooperation.
If formal means causal proximity, then that’s a whole different breed of cat.
I've argued precisely this, that the notion of "formal" meaning intention has been abused and misapplied now for a couple hundred years.
If you are a CAUSE of something, then you are a formal cooperator. Now, if it was not your will to do this, then you are not culpable or guilty of it. But you were still a formal cooperator.
I've used this example before. I put a loaded gun up to someone's head and pull the trigger, but in my mind I say "I do not intend for this person to die." Does that make you only a "material" participant in the person's death?
If I drive an abortion doctor to an appointment to perform an abortion, knowing full well that's what he's planning on doing, but I think to myself "well, I really don't agree with the abortion and wish he wouldn't go through with it." That doesn't make me just a material cooperator, since I am actually a CAUSE of the abortion about to take place. Now, if he could have gotten there some other way, e.g. calling an uber, then I was not a necessary cause, but I was still a sufficient cause. But if he had no other transportation options, then I become a necessary cause, a
sine qua non.
I believe that moral theology needs to be rethought regarding what is objectively sinful in terms of cause and effect. "Intention" has a place only in the internal forum. So, for example, if someone grabs my hand and makes me shoot that person in the head, then I didn't intend it. Objectively, my pulling the trigger however was a grave act, but I am not culpable for it. That should not, however, mitigate the OBJECTIVE gravity of what just happened ... it was still a murder and a grave matter.
So objective moral theology: cause + effect.
Subjective moral theology: intention + will.