I can make the following general observations. Any federal estate tax would be owed and paid for by the estate itself, not by your friend, There is a minimum estate value that the estate has to exceed before any estate tax kicks in. I do not know what that amount currently is because I have not checked it in many years, and it does change frequently. $5.4 million sounds about right, however.
As for any possible federal income tax liability, your friend most probably does not have any from the audio equipment sales. Your friend's basis in the audio equipment would be the fair market value of the property as of the decedent's death (or the alternate valuation date as determined by the executor). Since, as we Audiogoners know, the value of audio equipment almost always goes DOWN over time, it is extremely unlikely that there would be any capital gains, and hence no federal income taxes due from those sales.
Hope this helps. ---Steve