It does sound as though this woman took the poo accident (which is what it was) a little too seriously and ds picked up on it - what a pernickity woman. It isn't going to change the situation if you speak to her, I would just avoid letting your ds play there again.
I think you have got to try and be as relaxed as possible about the whole thing, he may be noticing that you are getting worried - try and make a joke of it if possible if he poos in the garden again, and gently say that next time how about we try and get one in the potty? Hate to say this but what about bribery, a sweetie for the next one in the potty!
Another thought - have you tried him on the toilet at all, does he have an adaptor seat? And has he ever see you or your dh going to the loo?
Hope this encourages you but our ds has just turned 3.5 and up until a few weeks ago was still insisting on nappies for poos, I was dreading nursery school on 1st Oct, thinking I would be called in to change a smelly nappy, but he has cracked it! We were on holiday with his older cousins (5&6) and I had been telling him every time I changed his nappy that they went on the big toilet for poos and the nappies would have run out when we got back from holiday. I stuck to my word and started putting him on the toilet when he wanted to poo - it was a struggle for nearly 2 weeks, he would hold on for as long as he could, running about holding his bottom, and getting a bit upset about sitting on the loo when I rushed upstairs with him, but I was very gentle and calm with him, and said that his tummy would feel much better when he had been, and he said that it did, afterwards.
It was like a little switch went on in his head -one day he was doing the above, and literally the next day he took himself off to the loo and proudly pooed and called me to show me, and did another 3 the same day all by himself, I was so proud and gobsmacked at the same time!
Sorry for rambling, hope that helps!