Thanks everyone,
His issue was not that I was likely to get into trouble because it was France or because I was a women, it's just that if something did go wrong, it would be much more complicated because I'm in France.
And to be clear, he never for a minute suggested I shouldn't go: his offer to go with me was genuine, even though he wasn't keen.
I think he and I have come to a good compromise. I'll cycle Dieppe while he catches the train. We'll meet in Lewes or Newhaven and catch the ferry together. We have both wanted to revisit a lovely Chateau we stayed in in 2008, and it's about 10km outside Dieppe, so that's perfect. The next day we'll do a relaxed 30-40km in the morning, lovely lunch then the same in the afternoon. We'll spend the night somewhere with a reliable train service to Paris. In the morning, I'll drop him at the station and cycle to Paris, and we'll meet there. Eurostar back on day 4. It ticks most of the boxes: I get to cycle to Paris. He gets the sort of weekend he enjoys, without straining too much.
Oh, and I am still planning to do the Paris in 24 hours, but now I'll wait until autumn, and I should have a road bike by then. He'll be more relaxed as I've done the route before, and I won't ask him to wait and worry in Paris. I'll call him when I get there, and pretty well turn around and catch a train the same evening.