While the ethics of his action are equivocal, Jimmy Valentine's plan to give the tools to a friend may be the only way that he can bear to part with them. Thus, his act becomes, perhaps, the lesser of two evils.
By giving them to a friend, there is also the possibility of retrieving them from this friend in the event that the engagement of Jimmy/Ralph and Annabel Adams does not result in marriage and a "happily ever after" life. After all, Jimmy's is the type of mind that considers variables and the odds of risks, etc.
Then, too, there may be enough of the criminal left in Jimmy that he feels the tools are so superior that it seems wasteful or somehow not sensible to just discard them; moreover, he may think that he might need them someday himself. Certainly, that Jimmy Valentine is yet a man who enjoys risks and opportunities for gain is evinced in his swift changing of persona so that he can attract Annabel:
Jimmy Valentine looked into her eyes, forgot what he was, and became another man.
And yet, Jimmy's character is more than superficial because he grows to love Annabel so wholeheartedly that he sacrifices his disguise and risks losing her respect for him in order to save her young relative's life. So, after his heroic actions and character transformation, a transformation that Ben Price recognizes and rewards, Jimmy may change his mind about sending the tools after all.