Hueyracer,
I have to disagree. Ian would have to have a valid, current EASA license to piggy back on. Think of the pain/expense associated with travelling to the UK, getting a medical, an examiner, paying the CAA, waiting, weather, etc.
Then he would still need a flight review in the US. All that would get him is a Part 61.75 private ticket that doesn't stand on its own. The Part 61.75 IFR rating requires a written test.
The validation is only good for 90 days. If he hasn't completed the commercial training within that 90-day window then he'll need a fresh validation before he can take the commercial checkride. (This happened to one of my students once.)
Or he can simply start the commercial training he needs anyway, take the private written and checkride along the way, and be done. Each point along the way is a valid ticket that's independent of everything else: real private, real IFR rating, and real commercial ticket. The instrument rating issued under Part 61.75 doesn't "count" for commercial anyway...
Terry