I've been heart rate training for a few years with very good results and I have some experience with overtraining.
Her iron/ferritin is definitely fine, so overtraining is the only other possibility unless she's getting sick.
First off...
What is her resting heart rate? Check to see if she has a more elevated resting HR than usual (she might already have)
Here's exactly what you need to do for her to recover properly:
Check her HR every morning, have her drink a glass of water (for hydration!)
and lay down for 5 minutes after waking up and check her pulse (have her count her heart beat for 1 minute)
She's already over-trained and likely sympathetically over-trained.
The heart-rate stuff above is optional but VERY useful in determining overtraining. A HR monitor would make this job much easier but I assume you don't have one.
First off.. Take the next day completely OFF!
Then have her run 30-60 minute easy SLOW runs the whole week. No hard runs, no intensity, not even strides.
And it has to be TRULY EASY...
She ran 18:55, which is around 6:00 mile pace. Her easy run is around 5K PACE + 3 Minutes, so around 9-9:30 minute mile pace, just for this week. Most people tend to run their easy pace too fast, trust me, I did too, and now it makes a HUGE difference when you slow down every once in a while.
Have her do an easy 75-90 minute long run that Saturday at the given pace, ONLY if her pulse has gone down. So 60 minutes on the days(mornings I recommend)she'd normally do her quality workouts, and 30 minutes on the normal easy days. Something like:
Mon: 30 Min Easy
Tue: 60 Min Easy
Wed: 30 Min Easy
Thu: 60 Min Easy
Fri: 30 Min Easy
Sat: 75-90 Min Easy
Sun: Rest
Her heart rate each morning should be lower and lower each day until it doesn't get any lower. That would be her normal resting HR. Once it doesn't get any lower for 2-3 days (it may vary by 1-3 beats). Keep her on this easy schedule until her HR CANNOT go any lower or you notice it stops getting lower each day. Think "recovery runs" are for recovering...this I call a "Recovery week," where she will recover from her overtraining and should be back to normal. NO RACING during this period if you truly want her to do well. Try this.
The tempo run where her "gears didn't shift..."
Easy one: Her legs are exhausted, demanding more oxygen than usual; higher HR from overtraining made her tempo more like a race effort since she went lactic much sooner, and consequently had a bad race as well.
Make sure that during this period:
-She sleeps well (BIG ONE right here.) She needs at least 7 hours a night. Have her sleep at the exact same time and get up at the same time. If she wakes up in the middle of the night a lot, that's also a bad sign only if she can't fall back asleep.
-She continues eating well.
-Learns to love running again
She'll be back and she'll have a big breakthrough if you do this right!
then start incorporating hard workouts, but only 2X a week and don't have her run them hard. Save the race effort for the RACES.
Make sure she goe