Reversal of the burden of proof in the event of imputed incapacity for driving because of disability ?

Moin, the following case from the acquaintance: A physically disabled person has been driving cars converted for decades for her. Of course accident-free and without points. Now came a letter from the road traffic office, created on the basis of an – as was stated – anonymous notice. This said that the person was accused “by observation” of “not being able to drive safely”. The person now has two possibilities: a) to sign the attached declaration, starting immediately voluntarily and to give up driving in the long term b) By means of a suitable, self commissioned report to prove his ability to drive. Is this normal in Germany, that I have to prove due to an anonymous message that an accusation does not apply ? Are there physically disabled users here with converted car and corresponding experiences ? Thank you !