Agreed, Feckless Clod is, well, feckless. He should be attacked on sight. I have no patience for clods like him. =D
It doesn't say that metagaming is against the rule explicitly anywhere, it's just a general consensus upon gamesmanship that metagaming is undesirable. While personal considerations are all very well in F2F games, in this new realm of online play there are a few notable differences. The most primary example of metagaming and perhaps the most obviously wrong is when two people playing here know each other in real life, and as such this drastically affects their play, to the point that say a husband and wife duo will never stab each other and essentially act as a super country. The other players in the game are at a great disadvantage because they don't have a similar relationship, nor should they be expected to. This is not to say that you cannot participate in a game with people you know in real life, it's just that you shouldn't be afraid to stab them or play them in the same way that you would an online stranger.
The problem extends to the issue in the OP. Perhaps you find someone that you work very well with, and so in other games you immediately ally yourself with this reliable person. This is the same sort of issue as you exclude the other players who may be just as qualified and dependable especially given that an individual's reliability is based more around the situation and opportunity rather than personal character. You put players who haven't had the opportunity to play with you in the past at a disadvantage which they don't deserve. This especially runs contrary to phpDip's mandate to spread the joy of Diplomacy to new players as it puts them at a disadvantage.
Sure it's ridiculous to ask someone to un-know what they've learned about other players, but I would definitely encourage them to try anyways. If it can't be altogether eliminated, it does not make the pursuit of it's minimization any less valuable. Be ready to stab your best friend, and co-operate, if perhaps grudgingly, with your nemesis. It's the spirit of Diplomacy.