Gardening Know How has a good article about this.

Because the tree bark function is to protect the layer that brings food, when tree bark is scratched or damaged, this tender phloem layer below is also damaged.

If the tree bark damage goes less than 25 percent of the way around the tree, the tree will be fine and will survive without a problem.

If the tree bark damage goes from 25 percent to 50 percent, the tree will suffer some damage but most likely will survive. Damage will appear in the form of lost leaves and dead branches. Wounds of this size need to be treated as soon as possible and should be watched carefully.

If the tree bark damage is greater than 50 percent, the life of the tree is at risk. You should call a tree care professional to help you repair the damage.

If the tree is damaged around 100 percent of the tree, this is called girdling. It is very difficult to save a tree with this much damage and the tree will most likely die. A tree care professional may try a method called repair grafting to bridge the gap in the bark and allow the tree to live long enough to repair itself.[1]

From the photo, it actually looks like the bark that came off was dead and decaying. It may well have fallen off on its own in time. That whole side of the tree looks unhealthy. But if it's still healthy enough, it will form a callus around the damage. The bark won't regrow, but any raw edges will scar and the tree should recover.

It's important to remember that trees aren't people. She shouldn't try to dress the wound. [2] If in doubt about the tree's recovery, best to call in a tree surgeon.

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