This is a serious situation and the honest assessment is that the odds of returning to prison are significant. Lifetime parole is exactly what it sounds like. It does not expire and it does not become less serious over time. Any new criminal conviction, including a DUI, is a violation of parole conditions that Alabama's parole board will have to address.
The DUI conviction in Tennessee creates two separate problems running simultaneously. The first is the Tennessee consequence, which he has already begun serving. The second is what Alabama does with the parole violation triggered by picking up a new conviction in another state. Interstate parole supervision operates under an agreement between states, and Alabama will be notified of the conviction. At that point Alabama's parole board has the authority to revoke his parole and order him back to serve whatever remaining time exists under the original Alabama sentence.
Whether that remaining time is life depends entirely on what the original Alabama sentence was and how much was suspended when parole was granted. Lifetime parole does not automatically mean a life sentence underneath it, but it can. If the original conviction carried a life sentence with the possibility of parole, revocation puts that full sentence back on the table.
The aggravating factors in the DUI matter considerably. A clean DUI with no injuries and no property damage is treated differently than one involving a crash, injuries to other people, or an elevated blood alcohol level. The more serious the DUI circumstances, the more aggressively Alabama is likely to respond to the violation.
He needs an attorney in both jurisdictions right now, someone in Tennessee handling the existing plea and someone in Alabama monitoring the parole situation and preparing for any revocation hearing. That hearing is where the outcome gets determined, and being represented by counsel who can make the case for continued parole rather than revocation is the most important variable left in his control.