This is not that uncommon.  The wheel bearings go bad and allow the hub to move back and forth on the spline when turning corners.  After a while you have worn hub splines.  Your friend should be replacing his bearings, not just tightening the nut as this may help his problem.  However if the hub splines are worn then he is probably going to have to look for a good, used hub and bearing housing to replace it so that he can get back to having that nut stay tight.  I had to do this on my '91 Vanagon so I would imagine that a '71 would be even more prone to it.

Hope this helps,
Ken Wilford
Van-Again
John 3:16
http://www.vanagain.com
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