i dont want to be a "that guy"

Im in a campaign where we just reached level 3, and one of the players is using a feature of its subclass 3 levels earlier and the DM seem to not know that feature is obtained at level 6. Im an experienced player compared to the rest or the DM itself, and they usually ask me about certain rules working or not working, but this time is not a rule but a class feature being used "wrong".

I dont feel like i should be that guy and tell them "hey you cant summon that thing because you are level 3 and you get it at level 6", should i just ignore this or should i point it out?

EDIT: The game came and i bring it up, i followed some of your advices and tried to be a bit innocent about it. Since we play in person with Roll20 up for maps and characters, my laptop is the one used to control every PC so about mid game we did a pause, and the DM told we already have control to summon the thing (Dancing Object for a Creation Bard), in which moment i said: oh, we can control this thing now? i thought it was later. Both the player and the DM were convinced it was a level 3 thing, but after a bit cold conversation we looked in the book and they were both confused because the statblock of the summon was above the level 6 feat so they thought it was a level 3 feat, althought the level 3 feat didnt mention the statblock. I think it went not as smoothly as your advices told, but it def wasn't a big big deal, but i could feel the disapointment in both DM and player. The rest of the game went nuts and we ended fighting a dragon, so i guess thats what they're going to remember. Thank you all for your advices.