I'm wondering if your tree has bacterial canker at the graft site.  Your friend may be correct in that the heavy branch may have caused a wound at the graft site that got infected. This disease affects up to 85% of cherry trees in our area. According to PNW Handbook: Symptoms The most conspicuous symptoms are cankers (localized areas of dead tree tissue), gum exudation, and dieback of girdled branches (girdled areas disrupt the flow of nutrients to the branch). Dead buds and leaf spots also can occur. In most cases, heavy gumming (a sticky substance oozing from the tree) is associated with bacterial canker formation on branches and twigs. Gumming occurs at the cankers' margins. Other problems can lead to gumming (see Cherry-Gumming).

Looking at your images, it looks like the graft area may have a canker.  Please take a look at these articles, which gives recommendations for managing this problem.
 http://hortsense.cahnrs.wsu.edu/Public/FactsheetWeb.aspx?ProblemId=34 . http://pnwhandbooks.org/plantdisease/cherry-prunus-spp-bacterial-canker

Please read the articles to determine if this is the cause of the branch die-back.