Steevo, whilst is is entirely legal for your prospective member to fly with the experienced friend I would ask you do consider a couple of points.
Firstly, insurance companies are in the business to make money. They have been known to get sticky in the event of a claim if they can get out of paying by claiming you didn't meet their requirements. If they won't give you their requirements in writing then go for the instructor option.
Second, the new member deserves a bomb proofing too. The syndicate member can certainly show him how the aircraft flies, possibly better than an instructor who knows the aircraft less well. The problem comes in knowing how far to let the new member go if the conversion is going badly.
Experienced pilot. 700 hours in similar types, or 700 hours in something completely different? I've flown with plenty of heavy type pilots who had great difficulty with the lack of inertia of a small span, lightweight aircraft.
In the grand scheme of things, and taking into account the proportionate cost over a few years flying of some time with an instructor, I would suggest that the instructor could save you money in the long term. You trained to fly the type with an instructor, if you are happy with the way the training went then why not get the same person to fly with your new member? If the new member is competent then it won't take long anyway.
Or change your insurers for a company who will say what they want.....
