They're cheaper than that, I'm sure there are plenty of donor vines he could snip off on the edges of the property...one of thē dirty secrets of that blackberry variety is that it is invasive, not native and will spread from any little piece of vine or root left in the ground. It is tolerated in many places because it optimizes food and cover for birds and animals as well as produces wonderful berries. Heck, i bet a bunch of the bushes will magically appear with no effort this spring because I doubt he thought about removing he rootstocks. If he did think ahead and roundupped them in the protected wetlands, it might show in soil samples. The blackberries, or lack thereof could be a potential lulz as either plays 'whack a mole' in his yard or gets written up as a polluter and still has to replace the horrible evil scratchy threats to his little ones (and the one barrier to them falling in the lake and getting trapped like his dog).
ETA: There are something like three or four other native blackberry 'relatives' here like thimbleberry and the ground black berry. They grow together in the wild. He's got a lot of replacing to do to get the balance back.
ah, missed
@Durable Mike Malloy and their succinct post. TLDR, what Mike said...