Beegee:
If, as you suggest, the planned holiday falls within final field training then it shouldn't be too difficult to take 3 weeks leave. It's all just a matter of timing though. If it happens to fall a few weeks after final checks then the chances would be slim to none and most would recommend NOT taking leave in the first few months after rating.
His best bet would be to make the trainee coordinator aware of the booked holiday early on at the academy, then they can make sure the information gets passed on to the relevant ATC group as soon as it's known where he will be heading so that final field training can be planned around it.
As far as practicality of it goes, taking leave prior to, or just after, the field sim would be ideal. Leave in the early stages of field training wouldn't be too disruptive, leave towards the end of field training would be..