The number will remain active for six months from the date of the last recharge of ten euros or more. The credit will expire, of course, but that can be recharged any time as long as the number is still valid.
If you know you are going to be returning to France after some time, you can buy enough recharge vouchers while still in France to (a) to recharge to keep the number alive while out of the country and (b) to top up a few days before you return so that internet and everything is working again when you get there.
If you didn't buy recharge vouchers when in France then Ebay seller emy351 on Ebay UK can sell them to you, but you have to pay a premium over the face value. (She has to make some money out of it.)
Or you could just loan the phone to your friend as you suggest.
