Is he allergic to all animals? Some kids are allergic to dogs and cats, but not guinea pigs or rabbits. It's terribly unfortunate if he can't be around anything furry at all.
It's a hard thing to explain to a little child, and there's probably nothing she can say that will end the conversation for good. It'll probably take him a little while to come to terms with the fact that cute little animals make him sick.
She might be able to try to get him interested in a hairless pet - a reptile or amphibian, for example.