Hello,
Did the client migrate to the Latest version of SEP 12.1?
SNAC is installed as part of SEP installation, so the version of SNAC is the same as SEP. When SEP is upgraded SNAC is upgraded as well.
SNAC gets activated in SEP when HI policy is applied on the client. Separate SNAC installation is not necessary. You can install the SNAC-only client (which has no AV/Firewall/IPS/Device Control, etc).
Check these Articles:
SNAC is installed on clients, but the service is not started, nor is the NAC component is showing on interface
http://www.symantec.com/docs/TECH178780
Planning for the installation of a LAN Enforcer appliance
http://www.symantec.com/docs/HOWTO81551
About installing an Enforcer appliance
http://www.symantec.com/docs/HOWTO55765
Installing an Enforcer appliance
http://www.symantec.com/docs/HOWTO81714
Does The Auto-Upgrade Feature of Symantec Endpoint Protection11 (SEP11) Support Moving An Agent From SNAC to a Protection Agent?
http://www.symantec.com/docs/TECH94353
Hope that helps!!