score:5
For that particular individual, if he was a serving RAF officer he would definitely have been debriefed on his return to the UK about his activities in France, before returning to active service. If the story of his involvement with the French Resistance is true, the there should absolutely be a record for it.
Details of his activities with the Resistance may have been recorded in his service record. The fact that he was shot down and managed to return to the UK would certainly have been recorded there.
RAF service records for the Second World War are still held by the MOD, and there is a procedure you have to follow to obtain a copy.
If the information from his debrief was recorded by another organisation (e.g. the Special Operations Executive or MI9), then you might be able to find it in the collections of the UK National Archives at Kew.
To locate records relating to the activities of the French Resistance during the Second World War held by the UK National Archives, your first step should be to search the online catalogue on their website.
Use the Advanced search option on the catalogue to search for the terms French and Resistance ("All of these words"):
Next, limit the date range to 1940-1950 (some of the records were created after the war when members of the French Resistance were interviewed):
Then click the Search button.
When I ran the search, it returned over 300 files:
NOTE:
Most of these records have not yet been digitised, and are therefore not available online. For those records, you will either need to visit the archive in person or arrange for someone to do so on your behalf.
More information is available on the National Archives website.
Good luck with your research.