Last shabbos a friend of mine suggested that using a kli sheni to make coffee on shabbos was unnecessary as the coffee has already been roasted, hence, will not be cooked further when mixed with hot water. Is that correct?
What he said is partially correct. It is not because it was roasted, rather because we are assuming that it was cooked after it was roasted, as there is “bishul achar tzlia” (cooking after roasting). Regarding the kli sheini, what your friend is saying is correct, that since it was already cooked it can’t be re-cooked. However the mishna berura says that any solid that will dissolve in hot water is considered a liquid, and that it is proper to be stringent with this for a kli roshon, and others say even for iruy kli rishon. However if the hot liquid is already inside to cup, you may put the instant coffee on top of it.
This that it is permitted to put the tea into the kli sheini, is the basic halacha, however the mishna berura says that it is preferable not to put coloring into water, because there is an opinion that it is dying, and even though we don’t pasken like this opinion, however he says that it is preferable to only put water on top of the coffee and not the coffee on the water. Therefore it is preferable to put the water into a kli sheishi, although it isn’t required. See Igros Moshe that this is what he did personally although it wasn’t required.
M”B 318-71 from Eliyahu Rabba, Shimras Shabbos Khilchoso 1-55. Igros Moshe O:Ch 4- 74 bishul 15..