From your description, I completely understand why you're concerned. It's much easier to ask the question now than regret it later.
As a surveyor, my first observation would be that the purpose of the external cill is to throw rainwater clear of the finished wall below. Ideally, you want a positive overhang with a drip so water falls away from the render rather than tracking back towards it.
With Rehau sliding doors (which are an excellent system), the largest standard PVC-U cill available is generally 225mm, so I would be interested to know why a 180mm cill was selected. There may be a perfectly valid reason based on the original survey or the construction of the opening, but it's certainly a question worth asking your installer.
The proposed angled trim may well work if it is mechanically secure and incorporates an effective drip detail, but personally I would want to understand exactly how it is being fixed. Simply bonding a trim onto the front edge of a cill relies heavily on adhesives and, over time, expansion, contraction and weathering can affect that bond. Ideally, you want a solution that is robust and permanent, not one that simply masks the issue.
That said, I certainly wouldn't recommend removing and refitting the entire door until you've established whether a simpler engineered solution will achieve the same result. If the trim provides a proper drip edge projecting beyond the finished render, it may perform perfectly well.
Before making a decision, I'd ask the installer two questions:
Why was a 225mm cill not suitable in this particular installation?
Can they show you exactly how the proposed trim will be fixed and how it prevents water tracking back onto the rendered wall?
A good installer should be happy to explain their reasoning.
If you could upload a photograph taken from the side showing the existing cill projection, together with one from the front, it would be much easier for those of us in the trade to give you more specific advice.
I hope that helps, and I wouldn't panic at this stage—I'd simply want to understand the proposed solution before any rendering is carried out.
