What happened?
A pax was restrained which only happens if they are endangering themselves or others.
Unfortunately restraints often result in injuries to both the crew and the restrained person.
Without photographs or video that's all it is- a claim that he was "beaten". More likely he got injured in the process of restraining which can happen if the person resists or is themselves violent.
Fact is- the pax must have been acting in a manner that endangered himself or the other pax. They don't restrain lightly and without a good reason IMHO. Restraints follow exhausting all other conflict management once those have failed.
Given the wording of the wife it seems she was not on the flight- crew do not "tie up" they handcuff.