I think our repair-odds stats, which tend to be overlooked in favor of the average repair frequencies, can be instructive here. We don't have these stats for the 2009, but do have them for the 2008, which should if anything be less reliable. As seen at the link, with the 2008 you have about a 55% chance of no repairs at all in a year. And that's before factoring in that your neighbor's car has been well-maintained and doesn't seem to have had any serious problems. This is much worse than the most reliable cars, where about 90 percent of the cars require no repairs in a given yaer. But not nearly 100 percent.
Another factor with European cars is that, when they do require repairs, these can be expensive. In the case of the 2008 3-Series, about half of the repairs are over $500, and about a quarter are over $1,000. Combine this data with the repair odds, and you'd essentially be rolling a die each year with a (roughly) one-in-eight shot of a repair over $1,000. The percentage going forward might be a bit higher, as the pie chart includes all reported repairs, not just recent ones. But it's clear that you're much more likely to have no significant problems with this car than an expensive one.
There are no guarantees. The car could turn out to require some expensive repairs over the next few years. But unless you're not comfortable with the moderate level or risk involved and absolutely want to avoid any problems it seems like a good bet. Especially since you know the car and owner.
To improve the odds, I'd have it inspected by someone who knows BMWs before buying it, just to make sure there aren't any lurking problems your neighbor simply isn't aware of.