They don't need an EASA licence, just an ICAO PPL to start the CPL flight training, as I understand it, so the UK PPL is fine.
I'd have thought that if they've passed the EASA writtens, then they should check if those are accepted by the UK, then look to use an FTO and examiner who are dual UK & EASA, and just put in two sets of paperwork. I think that EASA countries often (always?) do not accept flying training done outside of EASA countries, so those will probably have to be outside the UK - but I', unaware of any problems with the CAA accepting that.
I can't see any way around having two medicals eventually, but there are AMEs who are rated both with UK CAA and at least one EASA country (Centreline at Gatwick are one such, plus FAA, who I happen to know of), so that can again be one examination, then just two sets of paperwork.