I presume he will be spending more time in UK than in France? If that's the case, you're probably right. The Vodafone deals maybe best. i.e. Pick up UK Vodafone SIM's and activate Vodafone passport (for when they are in France) and if they are anticipating many calls with each other whilst they are in the UK, they could activate Vodafone Family. (This is all ofcourse presuming he can get a hold of 3 handsets)
I'll deal with Vodafone Family first. One person in the predesignated 'family' has to pay £5 out of his credit, and then all can talk to each other for free during that Month (for calls upto 1hr duration).
www.vodafone.co.uk/vodafonefamily
The passport option doesn't actually cost anything to setup (just dial 5555 from all 3 SIM's). With this option, when they are in France... each call (incoming or outgoing) costs £0.75 to setup then normal UK rates apply. This means calls from one person to another will cost a setup of £1.50 (the person who dials pays £0.75 and the person who recieves pays £0.75). The person who receives pays no further per minute rate, but the person dialing pays as if he is calling Vodafone to Vodafone back in the UK. (30p/min for the first 3 mins in a day, 10p/min after). Unfortunately Passport and Family do not work together!
www.vodafone.co.uk/passport