Anybody who thinks Marine will become President is a retard.
No one is suggesting that, but I'm saying she is a legitimate contender for the second round of voting. A Socialist could win, but the left has a bad habit of shooting themselves in the foot with presidential elections.
You're right, however when the far right makes it to the second round it is mostly due to vote split. After all, Le Pen's 2002 score wasn't that higher than his 1988 and 1995 performances. The main difference is that there were around 10 candidates these times, against 16 (!) in 2002. Sure, the FN has never been as strong as it is nowadays, but it's still "only" around 20%.
Also to note, a lot of Le Pen's support is due to her associating the usual xenophobe anti-immigration rhetoric, to a kind of left-wing populist position (anti-globalization, anti-corporations, etc...). It's certainly playing a "the left has betrayed the working class" game, which works well in France since the 1980. So, in some way, a lot of her voters can be considered "left-wing".