How can I help my cat get over the loss of his "friend" ?

I suggest you can adopt the new cat as she partner,I raised two cat in my home ,but at the first of i adopt a little cat,i afriad he alone ,then adopt the other one,they are paly together everyday.You can try it.Good luck!

Well, short of getting him another "friend" i would suggest spending extra time giving him love and maybe a few new toys to entertain him? I have somewhat deal with this before, when my female cat had kittens and they grew up and went off to new homes, she would walk around mewing and looking for them. What helped her get over the loss of her "babies" was usually a small stuffed animal that should would carry around in her mouth for a few days or so, pretending it was one of her kittens. After a period of time she got used to them not being around and was fine after that.