Over the years, I've heard some of the repair guys say that the positioning and tight space of the black Duos only made the problem worse.
I think this was probably half the issue. A lot of electronics made during this period died from bad caps. Heat most likely speeds up the failure process.
We'll eventually figure it out. Black Duos started dying...what, 2007 maybe? So the machines were basically 15 years old. If heat alone was the issue then we'll see them all die again eventually. Since my black Duo was still %100 functional when I sold it (right around that time, actually) and I would often leave the thing on for days at a time, I'm not a huge proponent of the heat theory. Also, Duos don't get hotter than the stated operating range of a cap, I'm pretty sure. The fact that lasted so long probably has to do with how much cooler they run than other electronics.
While I don't have any proof of this, I also feel that letting a system power down and stay that way for 10-15 years is also not great for caps. If this is a real thing, then it likely killed a few Duos since most got turned off in the mid 90s and didn't power up again until people started dumping them on ebay.
There really isn't any way to prove any of this, but anecdotal evidence on UNRESTORED systems is an avalanche in favor of the Duo R as the best overall system. When it comes to stuff that has been repaired, the IFU is actually a pretty good choice. This assumes the problem gear has been replaced.
In the end, OP, it doesn't really matter. Your friend will only own the machine for three days before he starts strategizing on how to sell it for max profit. In three months he won't even remember the thing. As long as it doesn't break during that period he'll have nothing to complain about.