I know that the book "Mine" is like the bible of resource guarding.
She needs to start by leaving the dog alone while he's eating, other than to toss extremely yummy treats from a distance at which the dog is still comfortable. As the dog continues to be comfortable (over days and weeks, not minutes) she can slowly move closer. Once she can be next to the dog, she can just do walk by's where she walks by and drops treats in the dog's bowl (without stopping - keep walking) as he's eating. Once he is starting to look happy about her approach, she can touch his bowl briefly as she's putting treats in. Then progress to holding the bowl while putting treats in, and then progress to picking the bowl up, putting in awesome treats and putting it back down. Again, this is a SUPER LONG PROCESS, weeks and months, but it's not something to skimp on. Doing this in a slow, patient fashion, should make a world of difference.
But seriously, buy "Mine" and find a good positive reinforcement trainer for individualized help.