Last year I was looking into this option myself, when I was in Brazzaville, and about to head North to Cameroon. According to local truck drivers, UN, IRC, and WHO, there is no road anymore. This road was open some 10 years ago, during the civil war, to allow
Timber export Via Douala port,in Cameroon. Now days the port of Pointe Noire is open, and there's no "need" to maintain this jungle road.
But there's only one way to find out...maybe with lots of time(2 weeks), and hard work it can be done again...
If your friend is riding the BMW R1200GSA, he is facing a huge challenge.
He can also take a barge from Ouesso to Braza, via the Sangha>Congo river. This trip down river is said to take 10 days or so. look at www.contrast.cx
Going via Gabon is also a possibility. Take the visa in Yaounde.
Hope this will help, and good luck. Your friend is now facing the hard part of the trip.