It is quite easy to tell when something is going wrong without using ACT. All you need to do is look at the speed the enemy's health bar is going down or the timer for the instance on the side of the screen. If you play the game properly, you will do enough damage to get through content at a reasonable pace. However, a no dps healer or someone who doesn't understand their rotation will cause significant delays.

Not telling someone how they can improve their gameplay is to the detriment of all, really. There are still level 80 tanks in Heroes Gauntlet who single pull and freak out when they are told to go faster. If I notice something going wrong, I try to offer basic advice like "use holy please" or "its ok to w2w here." It's not that complex and will do wonders to improve not just my experience but theirs as well, since they'll run into less friction later on now that they have a better grasp of what they should be doing. That being said, if they throw a tantrum or react aggressively to such advice, I will not carry that person through content. I'd simply leave without a word, which is allowed due it falling under a difference of playstyle.