An update.
Produkey worked, but we ended up not needing the key afterall. I sat down with my colleague and we worked through it together.
First of all tried the windows installation media from within Win10, where the first thing it does is ask for a key, which worked, but the actual install failed. Quick google told me we'd need to install it from boot. So did that. Went flawlessly and never asked for the key. Now back on Win 8.1