I think one of the best training devices for a fearful dog is a confident dog

Walking her with Jubel is a great confidence builder as she should see that he walks past people and sounds and sights without trouble and she can learn that she can walk like that too.
My last foster was skittish (although not reactive like Belle seems to be) and within about 3-4 days, she was looking over at Chester's reaction to things and walking much more calmly after she'd look at him and see that he wasn't caring a bit about the scary dog/truck/construction sign/etc
They've only been walking together for two days now as Jubel got sick the second night she was here and wasn't really feeling like a long walk Friday. But I'll see if she improves as the week goes on.
I would not be letting her charge over at Jubel and take back a toy/chewy. Jubel might be giving her a pass or just be a tolerant dog, but that is a really bad habit and could run her into trouble in her next home. I'd put a harness on her and leash and not let her have the chance to go towards him snarling. (Walgreens carries simple harnesses for about $6 that work great). Personally, I don't let dogs have any chew toys or treats in the same room. Those are for while they are crated, in different rooms or in different parts of the yard as I don't want to deal with any resource guarding fights.
She has a harness I keep on her when she isn't crated but if I left the leash on she'd chew it up. She's somewhat afraid of the backyard for some reason so I have to leash her to get her off the small porch to go potty. I learned the treats were a no-no last night. Started off fine having Jubel do something then a few repetitions of trying to teach her to sit then back to Jubel. Was fine for about 5 minutes and she decided she didn't like him getting treats too and started snapping at him, THAT got him to snap and snarl back at her but didn't actually touch her. Treats got put away and she went on time out for a few minutes in her crate.
I can stop her charging and snarling over chews most of the time but I never know when she's going to snarl when Jubel comes up to me to snuggle. She's been fine sitting in my lap as he hops up to sit with me and ran from the other end of the couch to bark other times. Don't know what sets her off as she's usually fine but sometimes not. Pretty sure I'm going to tell the rescue people I think she'd do better as the only dog, not sure if she does this with the Dobie her fosters have. Jubel might let her get away with it but I know a lot of dogs wouldn't. Got six more days to see if she improves I guess.