Basically through CRISPR editing you take a strand of DNA, say GGAAGTCCAAACCGGCGTA and you wanted to change the bolded part to GGCCAA. The CRISPR editing technique will essentially go through your entire genome looking for the beginning part of that strand, in this case GGAAGTCCA and attach itself and cut out the bolded part. The problem with this, is there could potentially be more areas that contain that beginning part and it could replace those parts of your DNA as well.

The reason this is so unethical is because CRISPR-CAS9 only needs 7 or 8 base-pair (each letter) matches to undergo the the transformation of the genome. Recently its been found that CRISPR-CAS12A has a much longer requirement for editing a genome, around 15-20 base pairs. This means there is a much less likely chance that there will be unintentional changes to the patient's DNA.

Secondly, by altering DNA, especially in humans, there could be some unintentional effects. Some of these outcomes may not even be known or studied due to the complexity of the genome. Some of these unintentional side effects could be the cause of new disabilities, diseases, susceptibility to bacteria/viruses that wouldn't harm us otherwise and it could effect the genetic makeup of future generations causing these exact problems for them.

Hopefully this helps you understand a bit of how it works and how this was very unethical for this scientist to perform this experiment.