@Hello99
Coffee grounds are a big machlokes haposkim.
First off, they are roasted before grinding; we know the machlokes about bishul achar afiyah.
Second, poskim discuss if it’s a food or a tavlin since can tavlin is permitted in a Klei sheini. See the ginas veradim who brings down this discussion.
And then, some poskim (I think including chasam sofer) say that even if it is a food, it’s already considered edible before adding the water since if necessary people would eat it straight. Thus it for sure would not be kalei habishul. (Rav mordechai eliyahu zatza’l relied on this, since soldiers in the field ate Turkish coffee straight when they didnt have water).
Plus, rav Moshe Feinstein zatza’l holds that in a Klei shlishi there’s no bishul even by kalei habishul, and the mishna berurah agrees in a case of bishul achar afiyah.
Lemaaseh some poskim (especially, but not only, sefardim) allow it in a kli sheini; some allow irui kli sheini; and some are machmir even in a kli shlishi.
So if the OP’ s rav told him he could rely on irui kli sheini, he definitely has what to rely on.