Not sure I'd do more than expressing my concerns as you will do, largely because it's hard to see what else you can do. Unless perhaps you have some sort of regular meeting of the building's inhabitants to discuss the maintenance shared spaces, and others share your concern.

Honestly, from what you said, the chances that she agrees to drop this plan of renting out the flat because of your concerns don't seem very good to me. (Unless it can be legally blocked by the local authorities as Avi argues, of course.) So if that's your definition of a good response, it may be a bit optimistic.

But if she expresses understanding for your concerns without agreeing to drop her plan, you could at least reach some agreements about what she is to tell her guests about the garden, and get some reassurances from her regarding screening of guests. By the way, possibly relevant question - when renting out the flat, where does she intend to stay herself? Anywhere nearby from where she could intervene in case of any incident?

If it just turns ugly, and you can't win either through local regulations or through the support from most of the other inhabitants, then I guess in your position I'd just try to make the best of it, and resign myself to simply watching Evelyn more closely and having possibly some occasional small discomfort from the presence of the guests. Going to the Justice of the Peace or something like that is a last resort if the discomfort turns out to be neither small nor occasional...