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The best you could hope for might have been to go halves on extending the party wall up.
A Party Wall Award will cover this (in kind), in that the person doing the work pays first pays for the wall, and the person enclosing upon it down the line will pay 1/2 the cost of the wall at current rates. Seems fair and by far offers the most robust solution. Those flashed-together dormer boxes with questionable separation leave a lot to be desired.
Well, it means that if you do the same, and have the horrible flashing bodge, then both you and the neighbour put themselves in to a world of hurt later down the line.
I wonder if you could actually prevent them from doing this, even with a surveyor acting in your interests. The best you could hope for might have been to go halves on extending the party wall up.
Friends of ours down the street agreed with their neighbour to split the costs of having it re-done properly when they converted theirs, so that now they have a brick firewall between them.