I would discourage the owner from trying to adjust this clock. There is a lot that can be messed up if one does not know what they are doing. My experience is that once setup if the clock stops keeping time that is an indication of some other problem.
I'm with RC. If the rate is off, just tell the custoer you will need to keep it a few weeks to adjust tje rate.
First thing, assuming the clock is fast by more than about 60 seconds per day, go ahead and move the adjuster (Bugs already mentioned) all the way to the -- . This trick will tell you if the rate CAN be adjusted.
Now, if it's fast for just seconds per day, nudge the adjustment finger a small amount to the -- each day until you get it close. Then, it will need to be adjusted weekly. Makeing very small adjustments at each weekly winding. + or -- one or two minutes per week is a reasonable target to shoot for.
Can't regulate it? = badly worn movement, back to what RC said. Willie X
You may get away for awhile with winding every 5 days but this is an 8-day clock which should at least run 9 or 10 days if it is really healthy. If it is completely down at 7 days you are only getting 70% performance. These are noted for issues caused by general wear through out the movement and in several places in particular. The expected useful life is something like 25 to 30 years depending on how the clock has been cared for. I expect that you will be seeing this one back before many more months.