I've never used a weed and feed spray around peonies but I've used the Ortho granular weed and feed in my garden beds before with no adverse effect on the peonies. Since the spray is an immediate action thing rather than time released you could be right. Hopefully someone on the forum will have more relevant feedback for you! My opinion is that whatever the cause, if the peony leafed out, regardless of curling leaves, it'll come back next year. Tell her to make sure it's getting watered frequently and the soil is well draining, no water sitting around in puddles on or really near the plant. If she can remove the damaged leaves without cutting back the entire plant that would be good. Cut the entire plant back in the fall and make sure to get rid of the damaged leaves entirely, don't leave them on the ground. If the entire growth is damaged she can cut it back now. The only real damage there is that it puts the plant into early dormancy. If you have cold followed by warmer cycles in the fall sometimes it'll put up new growth in the fall, it won't flower, though! This just steals energy from the roots before its real growth in the spring. Certainly not fatal but she won't get the growth she'd normally expect new spring.

If you have any pics of the leaf damage post them here, maybe someone will recognize the problem.