Hi Caroline - You mention that the architect is DPLG, which suggests to me that he is probably working regularly in France and must therefore be a member of the Ordre des Architectes. It's easy to check, just go to http://www.architectes.org/annuaire-des-architectes/l-annuaire-des-architectes/ and put in his name. If he is a member, and if he has acted in an unprofessional manner, then it is this organisation that would deal with any problems.
Does your friend have an architect's appointment contract? This would set out the architect's mission for the project and the extent of his involvement during the construction period. The contract would also have registry details for the Ordre des Architectes or ARB (UK), also the architect's insurance details.
Regarding the builder, do you have any details of his insurance whether in France or in the UK? There is no reason why you cannot approach his insurers just because he is involved in another dispute. Although if it is a UK insurer, it would be unclear whether he would be covered in France. When we have used UK based contractors in the past, they have specifically requested extensions to their insurance for European work. Again, is there a contract? Without it, any disputes would be difficult to regulate.
Good luck
Melanie