I don't bother commenting when someone already posted their new vehicle or have their heart set on one and are so excited about ruining their life with the better part of a decade financing an enormous SUV that they've posted pictures on FaceBook, all hope is already lost at this point anyway. If someone is simply asking about car buying advice, ill give it tactfully, but i'll only be brutally honest with someone who is a close friend and I'd only do that in person or a private message, not publicly under the photo of their new car or whatever.
I have a former classmate (we are not close) that has an absolutely horrendous financial history that they have shared over many posts on Facebook. I have never once commented or sent them a message about their many insane decisions, we aren't close and nothing I have to say will change their mind no matter how good the advice is. I've noticed over the years posts about only being able to pay off their debts if they win the lottery, posts complaining about their "loan place" (payday loan place), posts complaining about working too many shifts because they need the money, posts about the pile of new electronics they just bought (65" 4k TV etc), posts about how they can't get by with the amount of subsidy they get, and then theres the posts about their constantly revolving list of unmustachian, financed brand new trucks and SUVs that they are always under water on, which in the last year have included a brand new Jeep Cherokee, a brand new Dodge Ram 1500, and a brand new Dodge Durango, and then more posts about being broke. I'm not going to comment, I'm just also not going to congratulate them on their dumb choices.