If I am aiding an attack roll for my invisible ally which I made invisible but I can longer see him, can I still make an attack roll vs the monster to aid my ally? Or do I have to be able to see my ally? I would be using a whip to help aid him.
AID [reaction]
Trigger An ally is about to use an action that requires a skill check or attack roll.
Requirements The ally is willing to accept your aid, and you have prepared to help (see below).
You try to help your ally with a task. To use this reaction, you must first prepare to help, usually by using an action during your turn. You must explain to the GM exactly how you’re trying to help, and they determine whether you can Aid your ally. When you use your Aid reaction, attempt a skill check or attack roll of a type decided by the GM. The typical DC is 20, but the GM might adjust this DC for particularly hard or easy tasks. The GM can add any relevant traits to your preparatory action or to your Aid reaction depending on the situation, or even allow you to Aid checks other than skill checks and attack rolls.Critical Success You grant your ally a +2 circumstance bonus to the triggering check. If you’re a master with the check you attempted, the bonus is +3, and if you’re legendary, it’s +4.Success You grant your ally a +1 circumstance bonus to the triggering check.Critical Failure Your ally takes a –1 circumstance penalty to the triggering check.
Aid rules in PF2 are very hard to give an answer to in terms of "when/how can I aid by RAW?" because the RAW is basically that you can aid when your plan for how you are going to aid makes sense to the GM. So, the answer may very well be yes, especially if you and your ally have practiced some kind of signal (probably given by the one who is still visible) or have other good explanation for how this works. The answer may also be no.
Because this is a GM judgement call, I can only say how I would rule on a specific setup.