Could someone confirm my assumption about Btwin Original 520 Hybrid front wheel?
Your friend's bike will have the same size wheel on the front and back. If the rear wheel is 700c then the front wheel will be that size. The imperial equivalent of 700c would be 28 inches.

Finally, I'd suggest going to Decathlon or use their website and buying a replacement wheel with a dynamo. The price difference is minimal (less than a set of lights would cost).