Not a mentor roulette then.
So the guy wasn't expecting advice on gameplay - at all.
So bad context to give an advice, sadly for you.
Yes he is a lvl 70 and can't BLM.
But he didn't come for advices.
He came for the daily bonus, not to be told on his gameplay (which he's probably been told about more than once)
Yes, your advice wasn't welcomed at all in that context.
What would've I done ?
Stayed silent - and get a nice parse for myself.
And it already happened many times.
In the end, a "bad" DPS in a dungeon isn't actually a problem at all, less than when it is the tank or the healer.
In case of a less experienced healer or tank, I do with pots, self heal and aggro reducing skills...
If things really go bad, I will eventually drop a tanking/healing tip, but if I get a f- off in response, then I'll just take it on me and stop advising, or leave if it's really too bad.
I'll leave out all the "I am not here to carry noobs" talk... Idc at all...
edit :
So I just wanted to make sure - as a reminder this is what appears when you turn on the mentor icon :
https://ibb.co/cPVwtU
The second point says it all : be an examplar for player etiquette...
And most importantly : Yes bad mentor behavior might be reported.
Though I thought I heard somewhere about getting stripped of the title, it is not mentionned here at all, but I believe a temp ban from the title could be given.