Without reading the small print on your mother's policy, it's hard to give a definite answer.
However
(a) it's extremely unlikely that your nephew's equipment would be covered by your mother's policy ;
(b) if someone breaks into the outbuilding to steal his equipment, but also takes property belonging to your mother, the insurer could probably get out of paying out in respect of her property on the grounds that the intruder possibly wouldn't had broken in unless he suspected that he'd find your nephews property in there ;
(c) if your mother has ticked a box on her insurance proposal to the effect that "no part of the premises are used for business purposes", the insurer might then try to wriggle out of paying up against other claims she might make.
She should contact the insurer to let them know that your nephew is using the outbuildings. They might then say that, while his property isn't covered on her insurance, they'll continue to provide cover for unrelated claims.