MIMF

A friend of mine has a 6-str banjo where the saddle seems to be moving, constantly throwing the thing out of tune... is it common at all to glue the saddle to the resonator top, or is there another solution??

No, don't glue the saddle (bridge) to the top. It's meant to be held in place only by the pressure of the strings. If it's moving lengthwise (parallel to the strings), that means it's too loose. This could be caused by (1) the head needing to be tightened, (2) the neck angle needing to be reset, (3) the bridge being too low, or (4) some combination of the above. If it's moving sideways (perpendicular to the strings), that could be caused by the neck being at the wrong side-to-side angle (yaw). The latter problem is common to homemade 5-string banjos, because they have asymmetrical necks. Don't know about 6-strings.

The movement is lengthwise.. I have no experience with banjos - is there a way to tell if the "drum skin" is not tight enough , or how does one determine enough tension? ( - I was thinking about just fixing the bridge with a small strip of double sided tape... )

Many banjo tail pieces have a way to adjust the down bearing (increase it). If the head is too loose it should appear to be depressed below the rim at the bridge.

Banjo heads are designed to be able to "tune" the sound.
A tight fitting head will sound a bit brighter; a loose one warmer.
As John Clifford said, if the bridge is moving lengthwise along the strings, the head is most likely too loose.

There is a lot of good banjo setup information on Youtube.

Someone may have cleaned the head with some sort of furniture polish like Pledge. You might try cleaning the head and the feet on the bridge with naptha.