She needs to tell her vet that statements such as that will have the side effect of losing him custom.
That's just not true!
Sorry-not quite right, heres an extract from the Data sheet published by the makers
"Caution: DECTOMAX injectable solution has been developed specifically for use in cattle only. This product should not be used in other animal species because severe adverse reactions, including fatalities in dogs, may result. "
Basically if your vet uses an unlicensed product on your horse and your horse has an adverse reaction, your vet would be liable to legal action because he has used a drug offlicence.
Legally to even use the dectomax he should warn you of all potential complications, including death, and you should sign a disclaimer.
Yes 99.9% of horses will be fine with it but the odd one might not be and if your vet doesnt want to take that chance, hes not doing it to be vindictive or lose business! Just looking out for your interests-who knows perhaps he saw that 0.01% horse have a bad reaction.