I am a British architect and can speak with some authority on the matter as I have more than forty years exeperience, including much in the residential sector, and I also acted as a qualified expert witness in cases involving disputes between architects and their clients.
One would need to determine where the contract was made to determine where any potential legal case might exist.
Yes the professional qualifications of an architect would need to be verified. The governing body for the architectural profession in the UK is in fact ARB. They have a website with details of the complaints procedure. There is also a register of architects. An architect has to be registered with ARB and may be registered with the RIBA.
Architects used to be involved with projects all the way through a project under something called the full professional service. There are various work stages from inception (A) to completion (L). The service to be provided is agreed at the start of the project and the level of fees to be charged is also agreed (these are paid almost always in arrears unlike bulder who frequently demand up front payments on small project, this being something to be avoided and for which most architects will decline any responsibility). Ideally architects go through to the later work stages, including getting prices and then administering a building contract and approving payments and they should act as a quasi arbitrator on such stages. Therefore there should be no payment for work not done or incorrectly done.
A full service necessarily determines a higher level of fees than a partial service. During my professional career in London we were extremely hands on and visited site at least every week, providing minutes, taking photos, getting specialist prices etc. In all it was an extremely time consuming execise and the level at which we worked required, and got, fees of about 14% plus expenses and VAT. Most of our work was high end and as my firm reached nearly 40 staff at times over a very long period we undertook literally hundreds of private houses as well as larger scale projects. We did both new build and refurbishment including very many listed building projects.
However these days the process is frequently abridged and the architect is no longer involved on site. If he is not further involved then the risk is obviously much greater.
The building owner in this case needs to carefully consider their position and decide in their mind who is responsible for what. If they think after proper reflection that there is a case to be made against the architect or builder then that should be made in writing and the other person given the opportunity to respond. If there is no response then legal action needs to be taken against the parties and my guess in this case that it should be in France. Legal cases of this nature are usually long and expensive. An architect would normally have professional indemnity insurance and a term of the insurance is that they are not allowed to admit liabilty without the agreement of the insurers who basically take the case over.
Any complaint to a prefessional body is completely separate and will look at whether a professional job was done or not. Where a complaint goes against an architect that results in a fine or perhaps erasure (removal from the register) but does not deal with the matter of damages, which has to be done through the courts or by agreement with the insurers in the case of the architect.
Sadly bulders frequently disappear, or go bankrupt. Occasionally architects do but if they go bankrupt they will be erased.
I have very little experience of the French system as I am effectively retired and only do consultancy work for a UK company.
I also trained as a mediator which is a simpler process than running through lawyers and a court and would generally be preferable but all the parties neeed to agree to using the process. There is also a process called ajudication, and another called arbitration. As is the case with most disputes often the matter is not black and white but shades of grey. I have seen examples of clients deliberately using builder who are too cheap and cannot afford to finish a job, sacking them and then getting another builder in to finish the job off at a price much lower than should be the case. I have also seen unscrupulous builders and I have seen cases where an architect has failed.
Your friend may be sure that regrettably the process will be painful, expensive and long. I have always maintained that the building process is not for people of a nervous disposition. Over the years responsibilites have changed and with exposure on the media everybody is now an expert and a whole legion of project managers, cost engineers, interior designers, etc etc have sprung up. Architects have been paid less and less yet if anything goes wrong they are targetted because they have qualifications and insurance. Things are often not what they seem.
I would avoid offers of "free advice" and would recommend that proper advice is obtained on a paying basis. Otherwise your friend may end up going down the same road again. There have been lots of stories about notaires, estate agents and builders absconding around here, often with large sums of money, but I haven't as yet heard of any architect cases here.
Whilst people frequently scoff about architects and insult them we architects frequently have need to protect ourselves against rogue clients and builders. I could write a book about it!