I had the same problem a couple of months ago with my IS250,Nothing wrong for 12 months or so then came home from work to find it would not lock unless I held the fob next to button then following morning wouldn't recognise the key
spoke to Darryl at Derby who said Lexus are aware of this and they have an upgrade.
Two days later keys ,transponder etc all replaced under warranty
Since no problems (touch wood) so tell your friend to contact his dealer if it's a new GS300 should be done under warranty