I would think that it could be cut back gradually over 3-5 years. Be tough to make it look good during that process, but one might be able to make it look somewhat presentable. Are you sure this wasn't done intentionally or do you know these folks?

tj

I also know of a weeping cherry whose understock was allowed to grow out of control. It looks almost identical to the one pictured! I love showing it to people for their confused reaction to two different trees with one trunk. Then I take the opportunity to explain how cultivars are selected and grafted - a great educational tool!

I'd cut all the understock above-graft stem growth in one operation, rather than in stages.