What have they done so far? Did they keep them separate at first and try to introduce them to each other slowly? Slow introductions and patience are the key. If they didn't do this or if they missed some of the steps, they may be best to go back to step one.
I would urge them to read the following website which has some excellent suggestions that they might want to try:
"The absolute worse thing you can ever do is to bring home a new cat or kitten and toss them into the mix. All hell will break loose and injury can result. All this will accomplish is a lot of stress for all parties, and set an unfavorable tone for the future of the relationship between cats. New additions need to be introduced very slowly."
While each cat has his/her own distinct personality and it is possible this cat may never be all that sociable, there is definitely a good chance of improving the situation.
"The animals share with us the privilege of having a soul." -Pythagoras
"The soul is the same in all living creatures, although the body of each is different." -Hippocrates
"Let us be loving, hopeful and optimistic. And we’ll change the world." -Jack Layton
"Be the change you want to see in the world" -Gandhi
Kitties: Punky (13), Tweeky (6), and Sassy (7)
1991 to May 24, 2009