Mark them as “not spam” and the Akismet spam filter will learn, but it may take a few times of unspamming them before it takes and holds. This can sometimes happen when the spam filters have to make changes to counter new tactics by the spammer. They change tactics all the time.
If they are still getting caught after you approve them several times, have them contact Akismet directly at http://akismet.com/ and reference the URLs of a couple of posts they commented on where they got caught.
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