Well, Thoul, I must say that although I understand your friends predicament with the new cat, I am also highly disappointed to hear that he is already thinking of taking the cat back because "it just wasn't going to work out".
This is one reason why some agencies, etc., try to discourage people from giving pets as presents during the holidays, (Christmas, Easter, etc.). People are so excited to get baby animals on the holidays and then later find they can't take care of them for whatever reason and take them back, or get rid of them some how.
I think the reaction from the cat is normal since he has just been taken from his comfort zone and placed in a new home where he has no idea what's happening and who is there, etc. I think the wise decision would be to just take a little time and patience and see what happens. The cats may become friends after spending some time together, and maybe not. Only time will tell. If after a certain amount of time, the situation is still unbearable and no improvement shown, then it may be necessary to find the cat another home, but I hope that is done wisely where the cat will not have to suffer the turmoil of being accepted or rejected once again some where else.
In our own home, we have welcomed new cats to the fold that were not very companionable to begin with, but given time, they settled in and were accepted and now seem as much a part of the family as the others. Love, patience, food, security, and more love seem to work wonders many times. My best wishes to your friend and the cats - I hope they all work an agreeable relationship out somehow.