Seems to be the case also for non-admin user. Specifically, as non-admin user with floating_ips quota of 10 I can call 'nova floating-ip-create' more than 10 times. Error only when pool is exhausted.
Looks like the underlying issue here is that the relevant quota is the neutron quota, not the nova quota. These have different defaults, and on neutron it is 50.
Can you confirm that the customer is using neutron? If so, are they able to create floating ips beyond the neutron floating-ip limit?