A degree of actual physical presence is required under the underlying English common law, I stress here that the Wills Acts are interpreted against that background, not against that of the code civil.
If he is habitually resident in France at the time of decease, then technically it is the French Court or jurisdiction which has the jurisdiction over the entire succession, as they see it under article 4 of Regulation n° 650/2012.
If the testator is exercising the nationality option in the Succession regulation n° 650/2012, then in order to satisfy the French requirements, which will refer to the Regulation’s rules as to form, then article 22 3. reads:
“3. The substantive validity of the act whereby the choice of law was made shall be governed by the chosen law.”
I stress that fortunately in you scenario, the will will be an act but not an authenticated act whose signature is confirmed by an authority. In France , a notary would be the first port of call. However, do not neglect the huissier de justice…
It will need to comply with the Wills Act, and I do not see how Skype can be adduced to that to render the will valid signed under “virtual” witnessing under English law, which governs the substantive validity of the will.
I am unaware of any English caselaw which could stretch the principle that the witnesses have to be present to attest the signature and to apply their own signatures at a later date, which here physically would follow necessarily.
Reading between the lines, I would suggest that if the client wants to exclude otherwise forced heirs etc. by the nationality option, then he should ensure that the witnesses are there to physically witness his signature.
The substantive validity of the will could be set aside on an application by a forced heirs or other if they are not present in the room, and I doubt whether any Skype witness would be happy to perjure themselves to state that they were present.
There are presumptions in play here, see, for example, Sherrington and others v Sherrington -  All ER (D) 359 (Mar) as to presumptions as to intention on the part of the testator to execute, and also that the witnesses knew that the will whose signature which they were attesting was a will. However, importing English legal presumptions into France to face down a challenge which can allege that the witnesses were not present might defeat the entire object.
One point in Sherrington was the recourse to the presumption in the execution clause that the testator, who was a solicitor, and therefore knew that he was signing a will.
I would urge caution, and simply suggest that two persons be physically present who are told, in French, if necessary that they are witnessing the execution of an English will to ensure that they can say that the Testator signed it in the knowledge that it was his will.
I stress that French law does recognise the confidentiality of testamentary dispositions. May I suggest that a French notary be asked to provide two clerks for this, unless of course there are issues which the testator wishes to shield from any immediate view; either that or a French huissier de justice and their assistant could attest. That would be totally confidential.
Anyone up for drafting an express “virtual” signature and attestation clause incorporating ex post facto witness signatures so as to reinforce the presumption of valid signature? I would feel very uncomfortable, as the Wills Act simply iterates the basic common law position in statutory form, and I would not advise that a virtual presence of a witness, without their being able to actually handle the will on the signature of the testator and look at him and it, is the correct answer.
The real issue is cultural, whilst the French have an authentic acte or notarised will, the use of a typed format is less frequent, as the inevitable issue post death is that the disgruntled will say they did not know what he was signing, hence the unwitnessed French olographe will being the purest expression of handwritten and unwitnessed testamentary intention. Perhaps you could use one of those which technically can be valid as executed in accordance with the form in the state of its execution under the Wills Act?
You may need to consider what the effect of what is proposed, as a simple English “administrative” expedient, would be in France. To my mind, any disgruntled heir, or for that matter an administration would simply say that the document is a “fraude” on its face, and I have no doubt that such an argument would find a great deal of sympathy in France, and to my mind less than adequate support in England: the case of Sherrington being but one in point. How can the witnesses prove that they were in France without travel documentation? They cannot assert that the will which they will be asked to sign was the same one that they “visualised” the testator signing.
The difficulty to face will be that the document may need to be used in France, and it will not be seamless and watertight there.