Say a Clefable with Magic Guard got poisoned. It won't take damage, but Facade's power will double.
Hex and Venoshock
Both these moves gain the boost if a Magic Guard Pokemon is poisoned.
It won't be affected by other status moves
For instance, paralysis and burns will not affect a poisoned Pokemon with Magic Guard.
Heal Bell / Aromatherapy
Heal Bell still affects poisoned Pokemon with Magic Guard.
Psycho Shift still transfers the poisoning.
Hope I helped!