If someone has a PhD contract in France and wants to work during summer for instance, is it fine if that person receives any external support during that time, outside the PhD contract support?
It depends on the work you plan to do (see here, paragraph IX).
- You are NOT allowed to do extra activities of the kind that could be part of your PhD contract (teaching, diffusing scientific information, expertise...). If you want to do this kind of work, it needs to be under your PhD contract.
- For the other cases, you need to obtain an authorization from your employer. If your employer is the university, they will ask your advisor's opinion and evaluate whether the extra work risks compromizing your PhD before making a decision. I don't know if such authorizations are commonplace.
You most likely signed a contract for your research grant and/or a teaching assistant position. The conditions for a job during vacations should be listed in the part of the contract about vacations. Since there are many possible grants with their own conditions, it is difficult to be more precise. For example, as state workers, elementary schools teachers can have another work during their vacations, but with a lot of restrictions; I guess that state-funded grant have similar restrictions.
In any case, the first thing to do is to ask your PhD advisor, your doctoral school or the person in charge of contracts in your university administration.
I am not familiar with the French law, but if you are an employee and don't have an exclusivity contract, it should be fine. If you are funded by grants, they may require the student to not have other sources of income, in which case it will be stated on the conditions. I don't see any reasonable reason for this to be limited, but law works in mysterious ways.
If the person in this situation is already employed, the safest option is to contact the legal services of the university. If they are not easily available, a careful reading of the contract should do the trick.
If the person is thinking about applying for this, the eligibility conditions should state if the successful candidate cannot be working at the time of hiring.