The decision on whether a verbal consent is appropriate should be made by an independent body (not the investigator). Was the study approved by an REC/IRB?

If so, It would appropriate to inform the REC/IRB and ask their advice. The solution to solve this deficiency should be approved by the REC/IRB.

This is quite a real thing that happens but a written consent must be sorted with thumbprint if the person is illiterate in presence of a neutral witness . Videographic consents are better but as it can be edited so legal precedent can be difficult

now in this case if there is a witness that can help or an admission from the patient that they were explained evy

It seems to me that

1. if you informed correctly, they understood, had the opportunity to ask questions if needed, and then approved to participation, then their informed consent is valid from a moral point of view

2. Since the informed consent is not documented in any direct form (written, taped, video-recorded) this may still lead to trouble. There is no moral requirement to have the informed consent documented in a way that "proves" that it took place, but there may be legal or other formal requirements of this kind. So on the whole, it is always better to have the informed consent documented, perhaps with the exception of distributed surveys. Here it is fine to inform that "if you don't want to participate in this survey, don't return the questionnaire".

3. Depending on regulations in your country, and where you perform research, there can be situations where opt out (active no) is a feasible option. Then you inform, and tell that people who do not want to participate will have to say so. It is, of course advantageous if such opt out responses are documented.