Some cats will remain "difficult" their whole life...and I guess all Tome is trying to tell is that she wants to be an only cat...

Your friend can go through long turn methods, like keeping them seperate and introduce them again gradually, but Tome has clearly shown (as they have been seperated during their owners absence) that she's never fond of company...or they can try to medicate them...

In a tv magazine about pets I've recently seen a case of a cat behaviourist, where she recommended to give one cat into a new home, as some cats never would change their mind and their relation couldn't be fixed...and I think this is the same with Tome ...I guess she would only be happy as a single cat...

I am sorry, I have no different recommendation here...