So are you saying 'parking' for a pickup of less than 10 minutes is free?
According to sara3's link:
In addition to London Luton's Long Term car park, the airport offers the following parking, drop off and pick up options which are also detailed on the map below;
"1.Mid Term Car Park - FREE Drop Off, FREE Pick Up and FREE Parking for up to 60 minutes. 2 minute* FREE bus ride to the terminal.
2.Short Term Car Park -Drop Off, Pick Up and Parking £2.50 for 15 minutes, £4.50 for 30 minutes. FREE disabled parking for up to 60 minutes, 4 minute walk to the terminal.
3.Priority Set Down - Drop Off only. £1.00 for up to 10 minutes. Cars must not be left unattended. 2 minute walk to the terminal"
Was it defintely the free car park?
If so, then the only issue is whether he went through a no entry sign or contavened the rules in some other way. An appeal may be worth trying.