Well, if it's still very loose, I think you can reasonably conclude that either beef does not suit her or she has some other problem going on there.
I would certainly go back to the lamb mince, since you know that causes no dietary problems. If the loose stool clears up, then you've got a clear indication that beef does not agree with her. If it
doesn't clear up, then you have a good indication that there is some other problem (and having a vet check that out would be a good idea).
Working on the assumption that reverting to lamb clears everything up, then I think trying rabbit next is a good idea. Then perhaps another meat or fish, and then maybe giving the beef another try. If you get the same reaction from beef on the second attempt, then I think it's pretty conclusive that it doesn't suit her and has to be taken off the menu there
