This might be a totally stupid idea and I don't even know what the magnet collars look like (presume the magnet is seperate, hanging from the collar??)

Anyway, if that is the case maybe coating the metal magnet part in clear nail varnish might help with Lou Lou's allergy?

As I said, might be a stupid idea but thought I'd put it down anyway as I know people allergic to costume jewellery have used this trick with some success.