Helium isn't poisonous, but if you breath just it, you die suffocated I guess. There's a mixture of Helium and oxygen which is used in medicine to alleviate respiratory symptoms. On critical conditions it allows for easier ventilation while you try to make an emergency tracheotomy. The UCI in our hospital uses it in such circumstances, which gives some extra time. If he was asthmatic or had another lung condition, maybe he used this Heliox to ease his breath, and then would explain their finding helium in his body.