The only source of carbon debris produced in a turboshaft engine would be the carbon element of any shaft face seals.
Unless he is referring to "coke" which is pretty common in RR Allison engines and the Safran Arriel which we assume is in the 350 model in question. Rear bearing "low flow" which is normally associated with how you operate the thing - long ground idle periods - then shutdown soon after. People with RR Allison habits transferring them to the Safran engine is usually the problem. Don't let a Safran Arriel idle for more than 30 seconds. The rear end actually gets hotter and the oil flow is low resulting in "coking". Fix requires removal and cleaning.