This reminds me of some truly neglected & skinny(seriously skin & bone) horses I & a friend took on many years ago. We brought them home, with young foals at foot, and as they were also pretty feral & frightened of people, we kept them in a big yard/small paddock.
We had a vet booked, to check them out & advise the day after they arrived. On her advice, we fed them only grass hay to begin with, and not free-choice, but in a number of meals, in nets, so they didn't gorge too much after being starved. Advised to be VERY cautious and gradual in introducing more/richer feed. We also prepared to wean the (only 3mo & thin) foals too. Didn't do it immediately, for fear of further stress to the weak mares, but didn't want them sucking too much more from mares that couldn't afford it.
So... one morning, we arrived at the yards to feed, only to find them all hanging their heads & scouring. We didn't know why, but called the vet who advised this was a bit strange, could be bad & to withold feed, watch closely, call her out if they didn't soon improve(yes, in hindsight, a bit... strange advice, but was a long time before I knew it sounded so). So they did thankfully improve by later in the day. One mare was pregnant on arrival - found out because she lost the little-developed foal the next day though.
Then a few days later, the same 'sickness' happened again. And that the mares had been hanging out in a barren corner of the yards nearest the road. Went to see if there was anything apparent... and found traces of a bale of lucerne(alfalfa)!!! Some do-gooder had been throwing bales to our poorly girls over night and made them very sick, could very well have killed them!!!
That very day, we set up an electric fence along there, to make access by 'do-gooders' difficult, if not impossible. We also made big signs to say 'rescue horses. DO NOT FEED!'
John Doe may well not have been 'offended' by OP's behaviour, but frightened for the health of his horse if she were allowed to keep feeding him as she had done/indicated.