PRIM 1 FAULT prior to ETOPS entry, Reroute or Continue?
Since you are already airborne, the MMEL doesn't come into it. The system was normal when you dispatched.  What applies is the QRH (Quick Reference Handbook).  You should look for that particular message in the QRH in the section on status messages (I assume that fault messages are white status messages in Airbus-land).
If there are any restrictions on ETOPS operations with a fault that comes on while airborne, there will be a notation of some kind to that effect, referenced or attached to that message in the QRH, that you have to divert or take some sort of mitigating action before or after entering ETOPS conditions.  If not, you're good to go.
The MMEL is based on an analysis of risk exposure related to potential follow-on effects following a departure with a degraded system (that is, a second failure of the remaining component of the system in flight that renders the entire system inoperative or operationally compromised).  The risk level prior to departure is higher than the risk level while airborne, because as a general rule, double failures on the same flight are assumed to be extremely improbable.
And, obviously, you should consult with someone in authority at your airline instead of going on the statements of anonymous nobodies on the intertubes...