If they're doing the basics right, I'm good.
For tanks that means keeping aggro and picking up stray mobs within a reasonable time, dodging aoes and using def cds. If the tank actives Rampart right before running into a group, it's not optimal but I won't nitpick.
For healers that means keeping the party alive as long as the incoming damage is reasonable, dispell as neccessary, dodge aoes. I don't expect them to heal other's stupidity. If they can, good, but in the end it's up to the party to avoid avoidable damage.
For dps that means using aoe at 3+ mobs and single target for everything below that, dodging aoes and using their own dps buffs, even if timed a bit poorly like Raging Strikes towards the end of a fight.

If someone gets something fundamentally wrong like a RDM constantly hardcasting Impact/ Veraero/ Verthunder, A BLM standing around after MP are drained to wait for Astral Fire to fall off, a DNC not choosing a dance partner at 60+, a SMN not calling his pet, a tank not turning on tank stance even after the first pack of mobs had a field day whacking everyone but the tank, a healer spamming expensive heals without needing to and being out of MP halfway into the fight or trying to single target heal with Medica, I will politely speak up with something like "hey [name], don't you want to dance with me? I won't step on your toes, I promise!".
Yes, I've seen all of those. Not regulary but it does happen.

I think the moment you queue for something via DF you agree to teamplay similiar to a teamsport. Nobody likes their goalkeeper to stand at the side and pick his nose while the other team scores one goal after another. And they will speak up, even if it's just a casual game, even if they don't really know each other.
So I think the "nobody would do that in real life" is invalid.
I don't need to be a top football player to know that goalkeeper is supposed to stay close to the goal. Nor does somebody needs a WHM at 80 to tell me, that I shouldn't heal with Medica if the tank is the only one taking damage. If they know better than me, for whatever reason, I will heed their advice. The results speak for themselves and if they don't, I can always revert back to "my" way of doing things. No harm in trying something differently, if only briefly.

Nitpicking on minor things like less-than-optimal oGCD timing or prattling on and on about full rotations is imho pointless in DF. Keep it short, stick to the most imporant things and don't immediately jump in after one mistake. Fat fingering is a thing that happened to the best of us.
But if I notice someone seems receptive to advice, I will gladly stay behind as long as they are willing to listen and tell them what I know about the class they struggle with. But I won't force a full essay down anyone's throat who doesn't want advice.
If they're playing so poorly it prevents a decently smooth run and got aggressive after a polite attempt to give them a pointer, that's imho just as toxic as someone rudely commenting on poor gameplay and I will aim for a votekick. If that doesn't help, I will leave.