First, I'm so sorry for your loss. I can't imagine the shock and pain you're going though, and having your horsey haven becoming a place of stress can't be helping at all.
Secondly, while it didn't involve the death of a loved one, I once had a very similar problem a another border. I've had my gelding Avishay since he was 4 months old, and he's a "lifer" for me; I raised him, started him, and have done all his training. A few years ago, I boarded him for a few months at a large equestrian center to hold a stall for a friend while she was waiting for a horse that she was importing.
Down the barn isle, there was a woman who had a Hafflinger and a palomino quarter pony. I don't think I ever saw her ride but once, and it was, in a word, terrifying. Her horses regularly took her dirt skiiing, or simply broke away from her and ran loose around the barn. She was also convinced that her Haffie was palomino (genetically, that's impossible), and that my sorrel draft cross was ALSO a palomino. She became OBSESSED with owning my horse. I had other boarders telling me she would go in his stall with him when I wasn't there, and she even admitted to messing with him when I confronted her. She told me, to my face, that I was a "bad rider" who "made him unhappy" because I made him "work too hard" (aka I ride, lunge, handwalk, etc every day), and that he deserved to be with her.
At first I wrote off all of her early, seemingly harmless behavior as her just being a crazy woman. I know that you can't fix crazy, and I wasn't going to start stuff (I bear the double-edged sword of being a high-profile person in the local equestrian community because of my job). But when talking trash about me to other borders and slipping my horse a few carrots turned into going into his stall, bad-mouthing me openly, and stealing my stuff, I had more than one confrontation with her inside of about three days. Telling her to knock it off politely didn't work, and when I got more direct, she lost her mind and showed up at my normal barn time the next night with a briefcase full of cash (she was very wealthy) told me I could have $25,000 for my horse if I sold him to her right then and there. I had to tell her "No, you can't have him. He's not for sale" at least ten times. It ended with her screaming at me in the barn isle, and the words, "Someday, you'll be sorry. Someday, he'll be old and you won't be able to ride him anymore, and I'll be DEAD!"
I packed up my stuff, stayed at the barn until they locked the gates for the night, and was back with my trailer hitched when the opened the gates the next morning. I never saw her again, but I DID learn this:
The point of my story is this: It's easy to write off obsessive or pushy behavior, and say, "she's just a nut". I think it's easy to convince ourselves that another person is "harmless" or that we can handle things as they come up. But I've made the mistake of tolerating things I shouldn't have more than once, and ended up in sticky situations. I've learned that best intentions can go awry, and that it's always better to err on the side of caution. Do I think Crazy Palomino Lady would have hurt my horse? Probably not, because she wanted him so badly. But do I think she may have hurt me in some way after my rejection? Maybe, but unlikely. Do I think she may have hurt herself to gain sympathy? Possibly. But why stick around and find out? In my case, the barn had over 300 horses, a dozen trainers, and multiple owners and office staff. Going to someone with a complaint would have been next to useless. You seem to have some decent barn ownership and your barn seems small enough to make it so that something could happen if you made a complaint. I think that doing so would be in you and your horse's best interest. Alternatively, if you don't feel up to confronting her yourself, maybe you could ask your boyfriend to go with you and either back you up, or speak on your behalf (while you're present, so it doesn't look like he's acting without your support). But ultimately I think that as hard as it may be, finding a new barn may be the best, safest, and least stressful thing you could do right now.
My sincere best wishes to you!