Application group query information is captured in the system log only if it has been turned in the Application Group, the default is that query activity is not logged. The message numbers are 65 (before the query is run) and 226 (after the query is run, contains the number of hits returned).
Document retrieval information is logged by default, but can be turned off. The message number is 66.
All loading activity is logged. The message number is 87. This can also be determined from the System Load facility.
If this is a small customer with only one System Log table (SL2) then you can use SQL to analyze the activity. If they have multiple System Log tables then I think is easier to perform the analysis by searching the System Log using the client.
Using the client you could pick a date range, specify an In search for Msg Num with value 65 66 87, and a Like search for Message with value %<name of AG>%.
I don't know enough SQL to only return one row per AG per activity (query, retrieve, load), which is what you want.
As for leaving the Document Activity Tracking turned on in the AG permanently, I would say it depends on the existing workload on the system. Having the current information might be valuable to some customers, justifying the overhead.