The question points out that there may be potential harms or benefits to others besides the principals in the scenario, and that these should be considered as well. In the overall discussion, however, it is important to keep the principles of justice and individual rights in mind so that one does not just concentrate on ethical calculus. The means by which Eileen communicated her idea does make a difference because it bears on the issue of her right to receive credit for the idea. If it were a large, public forum, many in the field would know the idea was hers, and she would receive appropriate credit within that scientific community. That would be particularly true if there were some written record of her presentation of the idea in a technical note or poster abstract.
Publication makes an idea available to all to pursue as they wish.
Most researchers these days understand that anything presented in any form at a meeting may be pursued by anyone who learns of it. In this case, the context for Eileen's presentation was an informal, in-house seminar. In the past, I have usually heard this concept invoked by advisers who feel that their former students are talking too long to write up their research for publication. It would require a group familiar with the experimental systems involved. In this case, one year may be far too short if Eileen gets only one growing season per year for her experimental plant and plans her experiments one to two seasons ahead.