Is there any common word or expression for saying "to unload worker"? It doesn't matter what kind of workers they are - the point is - what's the right word for how to say "take some scope of work off some person"? I guess "unload" is inapplicable when talking about a person, or am I wrong?
The phrase lighten one's load is not uncommon.
The Synonym Finder lists Lighten one's load, lighten the load, or ease the load as synonyms for disburden, unload, assist, aid, unburden, and help.
Perhaps, the word "relieve" meaning 'to take the place of someone and continue doing their job or duties' would suit better:
I'm on duty until 2 p.m. and then Peter is coming to relieve me.
A part-time bookkeeper will relieve you of the burden of chasing unpaid invoices and paying bills. (=A part-time bookkeeper will take this unpleasant task from you.)
Unburden: relieve (someone) of a burden.
The new person was hired to "offload some work" from other people.
I can see why you would say or think 'unload' on a person's tasks because it is very, very common to use the metaphor, "crushed". For example, my developer is just getting crushed right now with google i/o coming up and their side hustle (moonlighting job) as CTO of a startup.
Perhaps a better phrase - de-task - unburden and my favorite jargon... - reduce their 'bandwidth'