Problem
In order to fully analyze this case we need to think about a couple things for example the issues related to holder-in-due-course (HDC), issues of promissory notes and various types of indorsements. We need to think about how the note was endorsed and who had the HDC rights etc.
Ok, let's take a look at the case. First Helen Pribus signed a promissory note to pay her son's debt. What we are not told is how this note was indorsed when Helen gave it to the Williams. However, most likely it was properly indorsed (signed by Helen Pribus,) It may have not been restricted that is why it could have been given to Bush for payment on the apartment.
Second we do not know why Helen Pribus refused to pay the note. However, the note itself, is an unconditional promise to pay on behalf of her son the owed to the Williams. Due to this analysis, this note is not restricted and seems to be a bearer paper. Whoever is legally bearing this paper (an HDC, for example Bush) can legally collect payment on the note, if all other issues prevail.
Third, recall that a promissory note is very flexible and Williams attached a letter denoting that the promissory note is to be considered for payment. It is not a qualified note. A note can be written in any form...Hence, Helen Pribus is liable to pay the note to Bush if he (Bush) is an HDC and if the note is properly indorsed!
The question is: Was it properly indorsed/endorsed?