I have been able to find the documentation that I was looking for. CDC is supported off of the readable secondary but Change Tracking is not unfortunately.
The link is specified below but here are the relevant parts for my needs.
Redirecting the Query Load to a Readable Secondary
While in many cases a client application will always want to connect to the current primary replica that is not the only way to leverage AlwaysOn availability groups. If an availability group is configured to support readable secondary replicas, change data can also be gathered from secondary nodes.
When an availability group is configured, the ALLOW_CONNECTIONS attribute associated with the SECONDARY_ROLE is used to specify the type of secondary access supported. If configured as ALL, all connections to the secondary will be allowed, but only those requiring read only access will succeed. If configured as READ_ONLY, it is necessary to specify read only intent when making the connection to the secondary database in order for the connection to succeed.
Change Tracking Note
Change tracking data must always be obtained from the primary replica. An attempt to access change data from a secondary replica will result in the following error:
Msg 22117, Level 16, State 1, Line1
For databases that are members of a secondary replica, change tracking is not supported. Run change tracking queries on the databases in the primary replica.
Replication, Change Tracking, Change Data Capture, and AlwaysOn Availability Groups (SQL Server)
Transactional Replication is typically used to off-load reporting to another server/instance and can be near real-time in a best case scenario. The benefit of Transactional Replication is that you can place different indexes on the subscriber(s) to optimize reporting. You can also choose to replicate only a portion of the data if only a subset is needed for reporting.
With Transactional Replication you will need Standard Edition or higher for the Publisher and Subscribers can be Express Edition or higher.
Have a look at Transactional Replication to get started.
If you have any questions, let me know. I hope this helps.
Best Answer
I can say with a very high level of confidence that there is no plan to support alternate indexes on AG secondaries of any kind in SQL Server 2016.
Perhaps your friend is confusing indexes and statistics - you can create alternate statistics on read-only secondaries (details here).