They could try French-Property.com, but it may not suit them unless they have someone that could take care of viewings, they’re not agents but certainly reach a huge audience & not just UK buyers.
If they want to advertise privately then this site gives a marketing route, Presumably if want to sell privately they have sorted out how to show it and get the translations and diagnostics done. If french:
These are only a few on Facebook: French Homes for sale, Property for sale in France, Expats going back to the U.K., Sell your house in France. The more the merrier. Others will pop up. They are free. There is a Dutch one which costs. to sell but free to look. HUISenAANBOD.nl.
I would give green-acres.com a go. Ok, it’s “international” but if its written in English, chances are English speakers will read it. Used to be about €100 for 6 months.
I think the pivot point in whichever methodology is chosen, relies on someone being able to show a prospective buyer around the property without too much fuss.
When we went to view some in May, the houses we had short listed from English websites, all had corresponding entries with French websites. The correspondence and arrangements were made with the English sites, but it was the French who actually took us round on our viewings.
May be an idea is to work backwards from who will take up the viewings, to then who will promote it ? But yes at face value… FB is a cheap start.
They might want to look at rightmove. Our British guests here in France quite often talk about having “the dream” of moving to France and rightmove is often one of the sites they watch.