Fillon is actually the most Catholic-friendly candidate out of the three - by a country mile. Indeed he’s the first candidate for the presidency to explicitly court the Catholic vote and play to the Catholic electoral constituency in the entire history of the Fifth French Republic, which is not known for such displays of religious affiliation given its strong secularist tradition.
Le Pen has views on abortion that would position her along Democrat lines in the U.S., despite being a far-right ideologue in terms of immigration, nationalism etc. She’s also a strong supporter of France’s famously (or infamously depending on your perspective) generous welfare state, another thing that wouldn’t sit well with the GOP or Trump. Out of the three main contenders, Le Pen is the only one not seriously pushing for market reform to make the French state less bloated and the economy more market-friendly to boost productivity. From a Catholic point of view, that’s probably the most admirable or at least inoffensive part of her otherswise toxic platform, but from a purely economic perspective she’s actually proposing nothing to help rejuvenate the flagging French economy - which needs liberalization. And from a conservative American view point, her pro-welfarism would conventionally be declared “socialist” and strongly condemned for creating a culture of dependency on the state.
Her niece Marion has actually fallen out with her over her “socially liberal” public stance on moral issues and Laicite, threatening to split the party. Marion has been losing the support of Catholic voters in her southern region of France to Fillon.
I doubt that many orthodox Catholics in France would advocate Le Pen - whose platform is very far indeed from both our moral and social doctrines in most respects - over Fillon. In France, they clearly aren’t - hence why she’s lost the “Catholic” vote, so to speak, by a wide margin to Fillon thus far.