Help needed

Getting in the system…

Hi I have a non French speaking friend in his late fifties he and his wife would like to opt into the French system they have no dependants and receive a small UK disability allowance, firstly what does he need from the Uk and the other Concern is would his disability allowance be affected as he would be resident here …any info would be gratefully received, as he is asking me for help and as I got into the system with different circumstances am clueless …t