When did your friends connection go in? When did yours go live?
I'd be amazed if Zen were doing FTTP migrations onto the BT Wholesale platform simply because it's far more expensive than using their own network. They don't seem to care that their own network seems to be broken. Chances are if your friend has been a customer for longer than a year then they are on BT Wholesale.
Have you tried using your friends PPPoE details and they try using yours?
There is some link between PPPoE details and whether you are on GEA or BTW - Zen have to make some server change when you move between them. I know this because twice I was migrated and the server change didn't happen, which left me without a working connection (as the authentication was set up on the other network still).
I'm not sure if that is the only thing tying the accounts to the service though, and whether it is possible to (if all other things are equal) move between services.
The OP could try to push Zen to sending one of the Samknows boxes to you to see how that behaves, I don't doubt it would also likely record poorer than expected performance. Mine was night and day when moving between Zen and BTW networks.
A symptom I noticed of the poor performance on the Zen GEA network was that most speedtest.net servers were poor, but there were a small number that were almost at line speed still. Swish fibre was one with OK performance, Voicehost (Norwich) was another, be interesting to know if any such difference is observed. Zen's own server was poor, as were most others. It's quite easy on the web interface or command line tool to specify particular servers.