Merck Veterinary Manual
Where it says in part, but please read the entire article:
Alsike clover ( Trifolium hybridium ) causes 2 syndromes in horses in the USA and Canada—photosensitivity (trifobiasis) and Alsike clover poisoning (“big liver disease”).
Alsike clover grows well on heavy clay soil, and an increased incidence of toxicity is reported during wet seasons. The disease is seen mostly when the blossom of the plant is eaten and the predominant forage being fed is the Alsike clover. The toxic principle is an unidentified phototoxin. Photosensitivity has been reported in horses, sheep, cattle, and pigs.
"thousands of dollars" is an awful lot to have spent on trying to find out why her horse keeps getting scratches
Nearly everyone on this forum has at some time, or currently is, dealing with scratches. Some of our horses have chronic issues. I don't think anyone can come close to spending that kind of money on vets for attempted diagnosis
Horses with chronic recurring issues, have a weakened immune system which should addressed. Some horses need a change of diet. All horses need their legs shaved, kept clean and topicals put on daily.
What topicals can also depend on the horse. We've all got our own concoctions that work for us. I use a 50-50 mix of hemeroid ointment & Triple antibiotic ointment. If the flies are bad I add SWAT or diaper rash cream to keep the flies off.
If I haven't caught the stuff in time (it seems to literally appear in the blink of an eye) and the sores really hurt the horse and seeping pretty bad, I put Dermalog on. Dermalog cost $79 for an 8 ounce bottle; expensive but still much cheaper than the thousands of dollars your friend has already spent
The liver damage referred to in the Merck link can also be a result of cellulitis that is sometimes a secondary to alsike clover poisoning.
My friend's horse had a severe case of cellulitis, several years back, that was caused by alsike clover poisoning, so it's nothing to mess with.
Hope this helps