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Lilies can be poisonous (all plants growing from bulbs are). I am not sure how bad just the petals are, though; I would expect them to be less bad than the green bits. it also depends on just how much.
Please keep an eye on your piggies and have them seen as an emergency if you notice anything untowards. Unfortunately, rodents can't vomit, so once eaten, it has to come out at the other end.
The pollen from some lillies can be deadly to cats, and lillies can also cause skin irritation to some people. For guinea pigs I do not know, but I agree - keep a very close eye. Does anyone else here go by Peter Gurney's books? Mine says crush a 100mg charcoal tablet in water and syringe it into your piggy's mouth, and an hour later give 1.5 ml of liquid paraffin, followed by a vitamin B injection (from the vet). The charcoal should absorb some of the toxins, and the paraffin helps to flush it out their back-end without the intestines absorbing too much of it.
Charcoal is best practice for poisons, however I do believe it would be worthless now. Charcoal inhibits absorption by the stomach mostly. But unfortunately as this was a few hours ago I don't believe it would make much difference.
If your worried I would suggest calling an out of hours vet service for advice on this.