I'd make sure they got regular water for the next couple weeks and mulch them really well, but not fertilize.

Yep- I agree- don't worry about how they look any more this year- by giving fertilizer now you would be asking it to put all it's energy into forcing green growth while tired and stressed- save those roots of the dehydrated plant to help it make it through the winter- I have a couple of painted fern that all dried up for the year already- if we have a long wet cool season I may see a bit of them again before next spring- I know I have to move them next spring as this will be the second (or more) year in a row that the neighbors Maple tree has sucked up everything I tried to give them-
It may not be something you can see above ground that is causing the difference of performance between the two plants- it may be (neighbors) roots- or rocks- or soil composition or pests- Or it might be a "sister" or "cousin" plant- different genitic make up as opposed to a clone- There are many fern's that go by the name of Maiden Hair- most are quite beautiful! It is nice to know you have a happy one too!
Good luck-
Julie

Make sure they are in full shade, also. All of mine are and they are looking fabulous! I haven't watered them and we have had precious little rain.

Linda

Thanks for the help!--I will hold off on the fertilizing. They don't get any full sun ('cept maybe around sunrise) and I water them pretty much every day. Now I am wondering if my spraying them with the hose each evening could actually contribute to the problem (eg, salts in the water, fungus, or something)? I do love these plants, and they have expanded nicely for me, but this is not the first year things have turned ugly following a heat wave.