Neighbor insulates the gable. Wants to insulate 15 cm on our roof

We do not tolerate a 15 cm encroachment on our roof

You didn't read or ignored my answer, didn't you? Practically, you have no say in NRW.
According to § 23 a Abs. 1 NachbG NRW, the neighbor must tolerate the encroachment if

[*]the construction measure on an existing building serves thermal insulation,
[*]comparable thermal insulation cannot be carried out in any other way with reasonable effort, and
[*]the encroachment does not or only insignificantly impairs the use of the neighboring property. A significant impairment is particularly assumed if the boundary is encroached upon by more than 0.25 m.

You did not read or ignored my answer, right? You practically have no say in NRW. According to § 23 a Abs. 1 NachbG NRW, the neighbor must tolerate the encroachment if

[*]the construction measure serves thermal insulation on an existing building,
[*]a comparable thermal insulation cannot be carried out in any other way with reasonable effort, and
[*]the encroachment does not or only insignificantly impair the use of the neighboring property. A significant impairment is assumed in particular if the boundary is overbuilt by more than 0.25 m.

We do not sign this formulation unless the takeover of dismantling and consequential costs is guaranteed for our own roof renovation. The roof connection from us is not possible to the planned insulation; dismantling would be necessary.
That costs

That is a significant impairment.

We do not sign this wording

You don't have to sign anything either. Unfortunately, you just have to tolerate it or take action against it.

Our roof connection is not possible with the planned insulation

Have this confirmed by an expert and your neighbor may have to plan and insulate differently (or pay for the dismantling).