Sounds like the perfect solution. Given that your ex-friend likes to sue people and all.
Otherwise I was going to say, I'm surprised you've kept it this long. I think you'd be well in the clear to sell it, give it away, burn it, toss it, whatever you want. (But I'd try to sell it and keep the money).
Or send her one more email saying, "The dress is outside by the trash bin if you want to pick it up." A similar technique got me tremedous results with the individual suddenly springing into action out of nowhere.
(My story: An individual who was privately employed as home help by my father, brought a whole load of her stuff/junk/furniture over and set it up in one of the spare bedrooms, claiming her place was too small. I believe she had ulterior unsavory motives for doing this but that's a whole other story. My father allowed this. However, once she was let go (for a multitude of reasons) I moved in to take care of him. Her stuff remained in that bedroom and I needed to 1) Use that room, and 2) clear out junk in preparation for selling the house. I sent her multiple emails and a couple of voice messages over the course of three months, asking her to come and get her stuff. She never responded. In the fourth month, I took all of it, save for one very large item, in the garage (really a carport, no walls), covered it in a tarp, and then I ran into her in town and told her. She said she'd be there the next day. She never came. Finally (weeks later), I took the large piece of furniture (very heavy, I needed a couple of guys to help me) and put it outside, in the snow. I sent her an email saying her large piece of furniture was outside and she could come and get it from there. Well. Seems this person DOES read her emails after all. She sprung into action, very angry that we'd put this item outside, claiming it had $1000 value, and "you could have at least put it up in the garage." Uh, yeah. You wanted us to break our backs hauling this thing up a long driveway in the snow. The very next day, she sent two men to come and get it. Amazing how easy it was for her once she thought this thing was going to get damaged. And I got another email later that day complaining that all the rest of her stuff, which had now been under a tarp in the carport for about a month, was "mildewy and damaged" and blamed me for not packing it better and "you could have at least moved it to the storage room." Our storage room. I replied to her email with a timeline of all the dates I'd asked her to come and pick up herself, and the date of the conversation in which she was told her stuff was under a tarp outside. I told her she was responsible for her own belongings, and that we were not a free storage facility. Her real motive, I think, was to try to get financial compensation for her "ruined" junk, because that is the kind of thing she does, according to everyone else in the small community. Just saying.....cause in the OP's case, she may find that as soon as this individual thinks her wedding dress is outside exposed to mud, rain, sleet, hail, and thieves, she might make a miraculous appearance.