The thing is - until the advent of the Habsburgs in Spain, the Castilians had been the traditional French ally south of the Pyrenees. It was Aragon, and Navarra that had disputes with the French. Castile had few disputes with France. If Juan of Asturias reigns on the throne, the main issue of contention will be the Aragonian territories (Naples and Sicily, to be precise), and how they are affected by the French conquests in Italy.
Off the top of my head, I remember Ferdinand splitting the Aragonian crown from that of Castile after the death of Isabella, to prevent Aragon from becoming the next Leon and falling into the hands of the Habsburgs, and marrying Germaine of Foix (niece of the then French king). ITTL, that is not likely to happen, and the interests of the French and the Aragonians are still going to clash in Italy. How much Juan is going to involve himself in the first Italian wars, supporting his father is open to question. At least, Castilian diplomatic and political support would be available to Ferdinand, I would imagine.
The major problem is that both the Habsburgs and Aragonians don't want to see the French in Italy (Aragon because the French had a claim on Naples, and the Habsburgs don't want the French in Italy, particularly Milan, since it is a backdoor into Austria) If they can coordinate their actions, it is going to be similar to IOTL, maybe even worse for the French, since two kings will be handling the affairs of the country separately and probably more efficiently. And given the dynastic connections between the Habsburgs and Juan, it is not even hard.
But if they cannot coordinate, then France is in for a windfall.