I'm sorry for your loss, it's always sad when you make a mistake like that.
I don't entirely know how a frog's mind works, but is there a chance your frog might not be as sad as you think? They aren't very social creatures in the wild. I'd have to do more research but I don't think their brains are as developed for bonding as mammals and other creatures. I hate to say anything about how an animal's mind operates with certainty though.
On the other hand, I have heard of a male frog that was restless and kept trying to get into a female's enclosure only to calm down when he was finally put inside.
If you do end up getting that 20 year old frog, I hope you'd let it spend some time in a separate tank first, in order to see if there are any symptoms of illness which could spread. Best of luck.
Also it would probably be best to look for a frog of the same species, because sometimes when different species come into physical contact for long periods of time the frog that has the more toxic skin can sicken the frog of the less toxic species.