Let’s work with this formulation:
1) A being with free will is free of any constraint including laws of nature when it comes to decision making on a situation.
There is just no reason to suppose that this is true. All that is necessary for a being to make a free decision in a given situation is a rational intellect (which we quite clearly have). Free will follows from knowledge and understanding, both of which intellectual beings clearly possess. A being with free will does not have to be free of any constraint, it must simply be in control, to some degree, of its causal activity. If we know and understand the effects of different actions, we can choose to perform one or the other based on that knowledge. There is nothing outside of us forcing us to do one or the other. We may have an internal inclination towards one or the other based on our emotions or physiology, but, except in cases of extreme mental impairment, we can still choose to act against such inclinations; a fact to which most everyone can attest.
2) A being that is free of any constraint is indefinable since it is free of any law that define its decision.
This, along with the rest of the argument, only follows if premise 1 is true. However, you have not provided any reason to think that premise 1 is true.