Kudos to your friend. Unfortunately, there are a lot of unscrupulous folks, and not just Thoroughbred breeders either.
A few weeks ago we had to put down our 29 year old retired racing Quarter Horse. Some folks would've sold her at auction a few years ago, but we wouldn't... she'd been a good broodmare for us, and deserved to have a decent retirement. It was a rough event though. She had a lot of heart and "there wasn't any quit in her"... so she was almost literally dead on her feet after the sedation but she wouldn't go down, and her trying to stay on her feet made it difficult to get the actual euthanasia drugs in her. It was all reflex and instinct at that deep level... I don't think she was really even concious, but it was hard... real hard. I was in tears, the vet tech nearly was, and the vet was upset... said it was the toughest one he'd ever done. Despite that, it was still the right thing to do, no doubt about it.
They do get drug tested.
That's for the horses that make it that far. A lot of horses don't ever get to the track due to injuries during training, starting gate accidents, etc. We've had a couple Thoroughbreds and several Quarter Horses with racing pedigrees. I won't go into details on some of the 'mistakes' that were made, but there are a couple of trainers we won't ever have anything to with again. It was ignorance or indifference, but either way, we won't repeat that mistake again.
... and Big Brown... go for it!