I'd suggest reading up on osteoporosis. That will give you some good, factual reference points for how fragile human bones become as their density decreases and they become progressively "hollow".
That would also seem to imply some changes to the underlying musculature would be necessary/expected, as lower density bones would not be able to hold up to the same muscle density that you find on the average human, nor the same amount of stress when moving around and articulating joints.
How hard would it be to break the bones?
One soarfolk to another? Probably about comparable to how hard it is for one human to break another human's bones. Their bones break more easily on an objective scale, but when you factor in the reduced mass and reduced muscle density (so on average a soarfolk is both lighter and weaker than a human) the subjective impression of how much effort is needed to break a bone is probably about the same.
A human vs. a soarfolk? Comparatively easy (relative to breaking the bone of another human), as the human will have stronger bones and greater muscle mass. I'd imagine most any blow delivered at full force would have a good chance of breaking a bone.
Would a human have to be careful about not pulling a soarfolk too
hard, hold him too tightly, etc?
Pulling I'd think would be okay except in extreme cases. The things likely to cause damage would be impact and torque. Holding/crushing might be an issue, but again I think only in extreme cases.
I'd think a soarfolk could generally shake hands with a human without fear, so long as the human didn't deliberately try to crush their hand. If the human did, however, they'd probably be able to crush a soarfolk hand (or ribcage) with effort.
Would slapping/punching cause severe damage?
Yes, punching especially. A human would be able to punch harder than a soarfolk, and would hit them with more mass than they'd generally be accustomed to. A human punching a soarfolk would probably be akin to a human with brass knuckles punching another human. Easily damaging, and potentially life-threatening in the case of, say, a strong blow to the face.