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In a situation like this, the first thing to do is to remove any source of contention. The person feeding the treats needs to stop. While they think they're doing something nice, they're putting both cats at risk of serious injury.I'd leave a letter (or talk to her if comfortable doing so) explaining this. If she continues, it consider contacting the rspca, simply so they can explain to her why she needs to stop.
I'd also contact the owner of the other cat if you can . You can work out a schedule so they're not outside at the same time.
You can also put a bell on your cat's collar so the other cat always knows he's approaching and can retreat quickly when needed.
How does this all sound?
I do hope this has helped and please let me know if you have any further questions.