France constantly flirts with the extreme right and has done so for many years. When it comes to the crunch the French hold their nose and vote for the alternative candidate. Every time the call gets a little closer, so maybe this time, it's time. I can't imagine a good outcome with Mr wishy-washy as the UK PM and Le Penn as the not quite opposite number.
The only benefit would be to soften the EU to a better deal for the UK, as Le Penn (like many) prefer the EU to be more like the EEA, rather than a superstate.