By going about it on a very analytical and methodical and rational way. Rationality is about the mechanism, not some particular moral or value content.
You might be asking "but if he was rational, couldn't he see that blackmailing is incompatible with friendship?". Well, if he wanted the trappings of friendship (having a friend group, playmates, hanging out and so on), and not some "genuine bonding", it's not that incompatible.