This might work for you when added to your code at the end -
hold on
for k = 1:numel(boundary)
plot(boundary{k}(:,2), boundary{k}(:,1), 'r', 'Linewidth', 3) %// Red border
end
Inspired from this blog
If you would like to keep the edge on the image saved for later usage, try this -
[M,N,C] = size(handles.ImageOne)
t1 = cell2mat(boundary);
ind1 = sub2ind([M N],t1(:,1),t1(:,2));
ind2 = bsxfun(@plus,ind1,[0:C-1].*(M*N));
handles.ImageOne(ind2)=0; %// Creates a black border