Ella Stuart, a polo groom for Nick Clague, said she saw the sandwich – which contains bacon – thrown into the field where the horses are kept.
Horses are herbivores – which means their digestive systems are not designed to process meat.
Ella, 20, said: “We love these animals – it’s a heartbreaking thing as well.
“They’re literally our lives. To have people not consider that it’s really heartbreaking.”
She added that she’s not angry about the situation but wants to make a plea for people to consider the horses’ safety.
If one of the animals was fed a food which it is allergic to, or one which its digestive system can’t cope with, it could be killed, she said.
Ella added: “You can literally kill them with one little mouthful of food.”
But as well as the emotional impact of a horse becoming sick it can also cost the owners large amounts of money.
Ella said: “The vet bills can be horrendous. It’s not just vet bills it’s the money you put into horses. We’re talking about thousands upon thousands of pounds.
“The upkeep. Stabling them. We feed them every day.
“On average per horse you pay about £500 a month to keep them healthy.”
Lucy Carmichael, a vet at Cliffe Equine Vets in Lewes, said horses have a very different gastrointestinal system to humans.
She added: “They’re very prone to getting food stuck in their throat – which can cause serious damage to their oesophagus.
“It’s just incredibly dangerous.”
Lucy warned that giving horses sugar-rich human food can risk upsetting the delicate balance of bacteria in their stomachs, which can lead to colic or laminitis – inflammation of their hoofs.
Ella said she wants to educate people about the dangers of feeding horses without their owners consent.
She also discouraged people from petting them and said if a horse becomes startled it could hurt itself or a person near it.
Ella said: “They’re our children. You would not want someone feeding your child.
“These are our animals and we would like people to leave them alone. We want people to be educated.”