I'm not sure a shield would work. There is a spell that might: protection from poison.
You touch a creature. If it is poisoned, you neutralize the poison. If more than one poison afflicts the target, you neutralize one poison that you know is present, or you neutralize one at random.
For the duration, the target has an advantage on saving throws against being poisoned, and it has resistance to poison damage.
If it is accidental, then he should just not use it in range of the party. If he is doing it on purpose... that's when unlawful and evil alignments come in handy.
Dwarves have "Dwarven Resilience", Stout Halflings have "Stout Resilience", and Monks have "Purity of Body" to protect against poison.