They call it snuggle flannel and then say it is not recommended for children's wear. DUH! I asked in JoANns because they have this flame retradant flannel but it also is not guaranteed to be completely non flamable. I do remember years ago there was a story about a little boy who was badly burned when his PJs caught fire because they were made of a blend and they melted on his body. Any cotton fabric will burn. In fact on the quilt shows they tell you to light a small scrap with a match to see if it is a natural fiber. Natural fibers will turn to ash while poly etc. will melt.
To be safe tell the parent that cotton flannel should not be worn around open flames. Hopefully by the time they are a parent they have that much common sense.