We have a 2100 cisco Wireless LAN Controller which manages 3 APs and assign IP address to clients which connects via wifi. Currently, we are deploying dual stack IPv6 and our consultant stated that our current controller does not support IPv6.

This is rather odd statement because there is an option to enable IPv6 in the advance tab of WLAN. And sometimes clients (win7 and Mac OS X) are getting v6 addresses, able to ping v6 sites, and reach v6 sites successfully. And other times, clients do not..or getting the v6 address for 10-15minutes, then unable to ping/reach v6 sites.

Our consultant is sticking by his words saying it does not support. If this is true, then clients should not be able to get v6 at all....not sometimes and not other times; very inconsistent.

And what is ipv6 pass through?
What is ipv6 bridging?

Could you kindly enlighten me and shed some insight? I'm not experience at all. So, any assistance is greatly appreciated.

Even with the latest software 7.0/7.1 WLC2100 is only able to do bridging for IPv6 traffic, which simply means that from WLC perspective there is no interaction with L3 information, just L2 forwarding of the frames with IPv6 as a payload.
Take a look on the "Guidelines for Using IPv6 Bridging" on 7.0 software http://www.cisco.com/en/US/docs/wireless/controller/7.0/configuration/guide/c70wlan.html#wp1345783

With software 7.2 there is a lot of add-ons for IPv6 support, however this software release is only available on 2500/5508/7500 controllers.