Don't worry, it's not going to make him sick or anything. The trouble with non-aquatic plants, like the peace lily, is that they rot when they are kept underwater. That's when the trouble starts.
I would take it out now and just keep it in a vase - no need to waste the plant, after all, and they are nice. :)
You can replace it with a silk plant, but hey, why not go with a live one? :) Or several... :p
Some good live ones include cabomba, wisteria, lacefern, hornwort, anacharis, java fern, java moss, anubias, ambulia or pennywort.
Stay away from anything that is purple, has striated leaves, or just looks like a land-plant. Some names to avoid, apart from peace lily, are aluminium, purple waffle, scarlet temple, mondo grass and (I think) Golden Ivy.
The more live plants the merrier, so I would as many as you can afford, of the ones I mentioned. :)