I find this highly concerning, biological organisms are orders of magnitude more complicated than man made hardware and software.
I think all he will achieve is either giving himself an infection or cancer.
Yes, highly concerning but not because biological organisms are more complicated, but because biological organisms reproduce. Any "hacking" mistake will reproduce for eternity.
Only changes to the gamete cells (sperm and eggs) will move on to the next generation.
You can change the DNA of specific cells. A change in you skin cell's DNA will not propagate to your bone cells. The skin cell has instructions to make a bone cell, but skin cells do not make use of that part of the code.
”The skin cell has instructions to make a bone cell, but skin cells do not make use of that part of the code.”
The scary thing is that it may require only a small change to change that. Muscle cells also have instructions to make bone and do not make use of that code, except when things go wrong as in https://en.wikipedia.org/wiki/Fibrodysplasia_ossificans_prog...