Yes, the computers pick the larger of the two mileages. A nice feature if you replace one with a new one as the mileage will be correct then, but annoying in this case. Well if your wife kills you, she will have no car and nobody to fix it, so...
After all, Homer Simpson said "better to ask for forgiveness than for permission"
Your friends' offer is probably not worth considering (despite his good intentions!) as he will need lots of "tools" and just because he's programmed a PIC doesn't mean he can find out how Renault programmed theirs. It's probably not even a PIC processor.
There are several versions of the "clip", and I think only the Renault dealers would have the version being able to do this, so buying your own would probably not help you as I would imagine the "good ones" are hard to find. They could also have a licensing system in them too which needs to be renewed from time to time, though I don't know the details here.
I think I would probably try to live with it. After all it's not something affecting how the car runs.
/Anders