Diff diff = ...;
Match match = diff.getMatch();
EObject leftChangedObject = match.getLeft();

ReferenceChange diff = ...;
if (diff.getReference().isContainment()) {
    diff.getMatch().getLeft(); // <= this is the left container of the added/removed object
    diff.getMatch().getComparison().getMatch(diff.getValue()).getLeft(); // <= This is the left version of the added/removed object
} else {
    diff.getMatch().getLeft(); // <= this is the object on which we've observed a change
    diff.getMatch().getComparison().getMatch(diff.getValue()).getLeft(); // <= and this is the left version of the object that's been added/removed from a reference of the previous object (object from the previous line)
}