Snail and garlic flavoured mixes should work well.Add a bit of vin rouge to get the final consistency.
Seriously though I'd have thought any mix that works well here should be equally good in La Belle France.Ask Malc for his recipe.
Au revoir mon ami.
I guess if they are taking all their bait with them and not buying any in France, then as noted by The Witness, all the usual baits they use in the UK ,would probably do the business. It may be worth checking the price for bait/groundbaits in France, because like many things, the prices in France can be cheaper than in UK.(if you dont mind stopping to shop!)