Can an ex-friend be sued for intentional damages or malice?

Yes !! It is possibly an intentional or ngeligent interference with economic or business propects. You'll have to prove malice but if you can show her bad faith because she knew you had created and were proceeding with the plan and she is thus doing it out of spite because you did not choose to use her "help", then you may be able to obtain a Temporary Restraining Order, a preliminary injunction and a permanent injunction preventing her from doing this within a reasonable time period before, on, and after the date you have contracted for. You started things first and took concrete steps to move the it along, including contracting for a location and catering, as well as other plans.