I am unsure of the legal position here, but I would suggest that your friend can deny employment, and is not obliged to offer a reason.
If he does offer employment, and has the conditions you advise, then your friend should be able to enforce them, as long as the same rule applies to all employees in a similar position - failure to comply could lead to a discrinination claim.
Are there reasons why the caps need to be in place - is the potential employee customer facing?
Your friend should have a reason why he would wish for the tunnels to be capped, and it should be reasonable, within th terms of the post offered.