Particularly for the return, this should be OK. At this stage it's just OLCI, which will happen, and there isn't a reconcilation approach during the boarding process. So long as you just leave the ticketing as it is, and don't do anything silly like apply for an FTV, then you should be fine. I imagine this is outwith the conditions of carriage, but they become unenforceable in this scenario. The non travelling passenger should not OLCI however, that is just creating extra problems.