he's got more issues than just morality here, as the DC rules of professional conduct for lawyers apply to him..

in part, he may be barred (or may at least need consent) from any litigation where his new clients interests are adverse to his former clients interests.

For instance... " a lawyer shall not represent a client with respect to a matter if: (1) That matter involves a specific party or parties and a position to be taken by that client in that matter is adverse to a position taken or to be taken by another client in the same matter even though that client is unrepresented or represented by a different lawyer; (2) Such representation will be or is likely to be adversely affected by representation of another client;"

See: http://www.dcbar.org/for_lawyers/ethics/legal_ethics/rules_o...