It’ll need a new nut and the bridge and saddle will need to be removed and replaced with one slanted the other way. Not an impossible task but unlikely to be cheap. I can’t be sure, but I suspect that the 12 strings are more heavily built than a six string so not an ideal conversion.

It depends how much nostalgia is invested in the guitar as to whether he would be better selling it and buying the 6 string he wants.