118b/3 A bowler is using the two-handed approach. The bowler’s dominate hand is the right
hand. Can the bowler use the right hand for spares?
Yes, since both deliveries are with the same dominate hand, the bowler did not change
delivery and, therefore, is not in violation of any USBC rules.
118b/4 A player started league bowling right-handed, but later decided to use a two-handed
approach. Is this a change of delivery and in violation of Rule 118b?
No, as long as all deliveries are made with the right hand, this would not be a
violation of Rule 118b.
118b/5 What are the requirements of a delivery for a two-handed approach?
A two-handed bowler is required to use the same dominant hand during their delivery on
both their first and second shots (this refers to the hand in which the bowling ball rests). They
also have the option of utilizing a thumb hole or not—both are acceptable. However, only one
set of gripping holes is allowed.

Thanks,  interesting however.  They should rethink that rule, as we all know that the two handed delivery is different from a lower rev one handed delivery.

Sounds like basic sandbagging to me: if the guy is intentionally using a release that he is not proficient in for one game and then switched to a release with which he consistently scores much better.

He is not violating the letter of the one law (using a single hand) , but is violating the spirit of second law: he is not trying to do his best on each and every shot.

The fact that the original poster says, "He does this on a weekly basis." indicates the guy knows he is ... not doing his best all the time.

Agree. It seems like this guy is trying to keep his average down for tournaments.