I guess there are times were no more can be done for a sick fish but I would be absolutely sure that I had used every option and medication available to me before considering putting the poor little thing out of his misery.
Being new to the hobby I have not come to this situation yet.
Boiling water ! I do not think that is correct I do remember reading somewhere on this forum that some prefer to use very very cold iced water I mean really cold ? But as I said I am new to the hobby hopefully someone else can give you more insight into this terrible subject.
Again I recommend you make sure you have done every thing else to try and save the little thing. What is wrong with him anyway ? and what treatments have you tried .
I recommend that if you want some advise on possible diagnosis you post your water stats i.e ammonia , PH , nitrIte and nitrAte along with your water temperature your cabinet details the symptoms of the sick fish a picture or two would help the people on here no end.
You may off posted this in the wrong section it may be better to post in the fish emergencies thread but hopefully a moderator will pick up on this.
Well good luck regards Ian.