Bantams coming into my garden

Could you perhaps go to her with a "concern" that you saw one of her chooks eating something in your garden that you thought might harm her birds. This might show her you care for her birds, but even she must realise that you are free to grow stuff in your garden, and it is HER responsibility to protect her chooks from eating anything nasty .

Or you could say you had recently used slug pellets or some other type thing and you were worried her birds might ingest them.  Might not be the most ethical thing, but she may then take more care of the birds wanderlust if she thinks they will come to harm rather than just seeing it as you telling her to keep the chooks her side
 

Glad to hear it's working!!!