I'm not 100% sure on how much customs tax a person in France would have to pay but the UK and other European countries *do* have to pay a customs tax , even on an item declared as a gift- if its over a certain value . The values differ from country to country, as do taxes. Euro and UK people dont get taxed if we send to each other, but if someone from outside of UK/Europe sends to us then we get a tax unfortuately. What makes it difficult in your case is that you are sending to a friend so you dont want them to ave a hefty fee to pay before they can get their parcel from you... but on the other hand you dont want to undervalue it either because of the insurance incase it went MIA.
I'll see what i can find out about customs tax in France, and in the meantime maybe a French member can shed some more light on the subject too