If he was still on probation I see no basis for any statutory limit. There is considerable risk here, and I think he needs to hire an attorney to try to work this out with the PO or the court.
You can search out a local criminal defense attorney here on AVVO.
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If the probation violation motion was filed before his case was closed, then his case stays open and there is no limitation on the State’s ability to try and revoke his probation and resentence him. If his probation has not yet been revoked however, he can still fight the probation revocation motion and might even have a chance to win. Further, there may well be a chance to work out a new plea bargain that would keep your friend out of jail. Your friend should seek legal counsel as soon as possible.
Legal disclaimer: The above answer is intended to provide general information only, and does constitute legal advice. No privilege is established by reason of having answered the above question. We do not have an attorney-client relationship until you make an appointment, we discuss your case face to face, I accept a retainer, and we explicitly agree to enter into representation.
Missing a meeting with a PO is grounds for a violation of probation which then puts you before the same court for resentencing on the original charge. Are you sure he wasn't violated for leaving the state without permission, it is very rare for a PO to violate you on one missed meeting in CO
PS there is no statute of limitations - once his parole/probation is over, he can't be violated but the record of his felony stays on his record