For some reason, the wording that explicitly forbids using out-of-play balls to measure distance isn't copied/pasted into the WPA rulebook. But the BCA ruleset (which is where the WPA ruleset originally was copy and pasted) still has it:
http://bca-pool.com/general/custom.asp?page=55
1.3 USE OF EQUIPMENT
Players may not use equipment or accessory items for purposes or in a manner other than those for which the items were intended (refer to rules 3.42 and 3.43). For example, powder containers, chalk cubes, etc., may not be used to prop up a mechanical bridge (or natural hand bridge); no more than two mechanical bridges may be used at one time, nor may they be used to support anything other than the cue shaft.
Extra or out-of-play balls may not be used by players to check clearance or for any other reason (except to lag for break); the triangle may be employed to ascertain whether a ball is in the rack when a match is unofficiated and the table has not been pencil marked around the triangle area. (Also see Rule 2.3)
2.15 INAPPROPRIATE USE OF EQUIPMENT
The referee should be alert for a player using equipment or accessory items for purposes or in a manner other than those for which they were intended, or for the use of illegal equipment, as defined under “equipment specification”.
Generally no penalty is applied. However, should a player persist in such activity or use of such equipment, after having been advised that such activity or use is not permissible, the referee or other tournament official may take action as appropriate under the provisions of “Unsportsmanlike Conduct” (Also see Rule 1.3 and Rule 1.4)
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Reading these both, it reads as if a warning should have been given, not a foul.
It's still bizarre for Dennis, but it looks like he go the worst of it.