Answer: Since yellow is a fairly unusual color in hellebores, it is most likely a hybrid. The majority of hybrids are varieties of H. orientalis. There are so many different hellebores on the market now, and they hybridize readily too so it might have been a seedling. One of the better known yellows that can bloom through late spring/early summer for example is Heronswood Yellow (from Heronswood Nursery.)
Just to give you an idea of the range, here is some basic overall information on hellebores including some of the species and varieties. You may need to cut and paste the complete url into your browser to make it work correctly.
You might ask the property owner if they know what it is specifically, or you could check in a reference source such as Graham Rice's book(s) on hellebores. His hellebores web site includes some information you may find helpful with regard to other books on hellebores as well as sources (including Heronswood), perhaps a check of the suppliers he lists would yield a similar plant.
I hope this helps.
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