I think it could very well indicate a MAF problem... at startup your car doesn't go into closed loop rigth away. If he gets a good idle /w no maf before it warms up and it idles like poo /w the maf then I would clean or replace the MAF befor anything else.
I may have missunderstood though, is this a problem that only occurs after the car warms up?
COULD is the important word there.
When the computer doesn't see a MAF input signal it changes strategy. Regardless of OL/CL.
The described condition does not exclusively point to the MAF meter.
jason