Casual and seasonal employment in France - requirements?

We live in the UK and have been invited to do a six month house-sitting in France from March until September next year and would love to accept the opportunity. The house owners have said there could be some part-time decorating work available on the house for my husband Chris (who has his own artisan decorating company in the UK), while we’re there, and they would like to pay him to carry this out.

However, we are unsure on the French law / rules for seasonal / temporary / casual work of this nature. Is there a maximum number of hours he could work / euros he could earn on a casual basis?

Would he need to acquire any formal employment status to enable this arrangement? If so, how would he go about it? The response to these questions will determine whether we accept the offer or not, as we could not afford to be there for six months without Chris having an income of some kind.

It is our intention to explore different areas of France with a view to moving over to settle somewhere, at which point Chris would apply for his SIRET number, but just need some advice on how he can work a little in the interim. Any signposting to appropriate websites or other information would be gratefully received.