Do you need regulations to justify a safe training?
The question before a solo flight is simple. The answer needs competence and experience.
Question: Would this flight be safe? Is he capable under the given conditions?
If you are not sure, investigate more, there are unlimited possibilities to achieve the standard safety level.
I had students with pass theory and radio license, but not capable to handle the communication on medium buy airfield? Yes, they were sent solo, but with conditions they could handle safe.
Some had also theory and radio telephony, but never reached the level for the skill test.
Of course, is involving of other instructors a helpful approach, either to get their blockage removed, or to confirm to stop for the skill test.
Conclusion:
Instructor competence ca be built up with help of others, regulations often used to be safe on the legal side, if something happens.
Do everything reasonable, that nothing happens is the better approach.
If you are not sure, as your mirror image and be honest to yourself.

Interesting fine views ,however I am interested in the legal part.

I did my first solo in a Cessna 150 aeroplane in March 1968 without a Flight Radio Operator Licence (FRTO) which was officially issued when the Private Pilot's Licence was given in August that year.

Hello,
In France, on this point, strict EASA regulation are in force. Nothing more.
In my flying club (DTO), we ask before solo navigation to have theoritical (PPL or LAPL). But it's more than what as EASA. However lot of training organization do that (for motivation and knowledge purposes).
Antoine

Hello,
there is no legal requirement for first solo to pass the pre-exam school tests for theory (75%) here, radio license is required however due to other then Aviation law. Student pilot is flying under Student-Pilot License and FI is responsible for evaluation of his level of preparation till final PPL Theory Exam and Skill test.
Final exam test procedure for theory at CAA can be performed any time during flight training or at the end, planning is matter of student, when theory study is finished.

Thank you ofr your valuable input - it is as expected very different interpetations in different countries. Seem like Danish CAA entepret the rules (or their authority) quite strickt. Thanks all