Bringing in his machine won't conclusively prove it to them - what if there is an issue with his power supply for example? They don't know if there is or not.
They obviously want to verify in a known testbed, maybe in UK if PC world or the likes (which explains the 10 days), as they may not have a machine they can slot the motherboard into onsite, or the 'expertise' to do so.
For all they know he could have blown it when attempting to install, or just changed his mind and pretended it wasn't working?