JohnP,
Plugged balls are now allowed on tour, including Regional and Senior events. This rule changed 4-5 years ago. They first allowed clear plug only and something like only an 1/8" could be showing then a couple years later did away with it altogether.
That being said, he's actually bowling the Pro-Am not the actual Regional, so it'd be fine either way.
To Teenbowler,
Depending on how close the span is, the other option would be to change the grips then maybe just use an 1 3/8" thumb slug to manage the rest of the change. However if the span isn't within 1/4" you'd definitely have to plug the thumb. If it's just the thumb that needs to be plugged and redrilled it shouldn't be too bad and you should be able to find someone that will do it. As said though, some shop guys are dead set against working with Rubber Balls. Usually the worst that happens is the plug with crack.
Overall, it might just benefit you more to buy a new plastic ball, overall it'll be a little more costly, but most likely less aggravation.