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Typically, the DA will normally notify probation in the other county once they receive his RAP sheet and realize he's on probation elsewhere. If not, probation would ultimately be very likely to find out anyway. It is typical of probation to run a probationer's criminal history before the termination date of his probation. This is done specifically to make sure that nothing like this has occurred and that the probationer is eligible for a favorable termination.
You can expect that sooner or later probation will find out if he doesn't tell them himself, which, I'm sure he knows, he is supposed to do.
Both arrests and what happens on the case will be on his criminal record. If the county never looks up his record, they won't see it. But as I've said, generally the DA will notify probation in the first place and if they don't, probation is known to check to look for things like this.
Nobody will be able to guarantee you that he will be safe if he fails to report the new arrest himself. Could it happen? Yes. Will it? The odds are better that each county will eventually know about the other case than that they won't.