no, not at all. but if you dont care about the electrical bill, then gohead and leave it on, but it will raise your bill [slightly]. tho, after time it will loose stability (like over the course of a few months) from old stuff that is in the ram, so pretty much, the longer it is on, the slower it gets untill you reboot. and its smart to (and any other time) regulary scan for viruses, defrag, and check the disk.
ps tell your freind to turn off the pc and clean it out, i imagine that there is like 1cm of dust inside. and with the increased on time of the pc, it will accumulate dust faster inside the pc, so if you plan to have it on as long as your freind, get some fan filters for it.