As far as I know a peace lily is a spathiphyllum, a plant often used in Betta bowls. Its roots can tolerate being submerged in water, hence the reason they are used in such fashion. Normally, it is kept as a house plant, with its roots in soil, sphagnum, and/or ground coconut shell.

To answer your question, yes, it is safe to use. I have one in my dart tank.