You can also turn Siri off in "restrictions" - Go to Settings > Restrictions > Siri and make sure it is ON if your friend has enabled restrictions in the first place.
If you are not able to change those options it is also possible that your friends phone has a provisioning profile which prevents Siri from being used. The most common reason for having one of these is because the phone is supplied by his place of work and they do not allow this feature to be used as a general policy.