It's a tough one. Toadmum is right, in theory if you put a Bexley school first it should become obsolete as he didn't qualify, however I'd be very careful. I know last year, first hand of someone who did something similar. Their daughter missed Bexley, but passed Kent. They put Kent school second, lived out of borough, but not that far, but didn't get the Bexley or Kent school (was offered a school miles away). whereas the girls best friend, who lived further away, put the Kent school first and got it.
Based on this I would always put Kent first to ensure a place, it depends on whether they want to take the risk?
How far away do they live and which Bexley school are they after? Seldom are Bexley appeals successful unfortunately.
There must have been some other factor you are not aware of - if the child missed out on Bexley but was eligible
for the kent school then she would have got it. Maybe she actually had another school second and Kent GS third?