Draco knows that HJPEV claims to be able to Patronus. Draco knows that HJPEV presented himself as though he needed to hide his Patronus. Lucius knows that the 'light' side regards the ability to Patronus as a 'light' qualifier.

If Lucius also knows what Draco knows, then he would know that inviting HJPEV to Patronus would probably result in one of two things: either he learns something HJPEV may believe to be a valuable secret, or he casts doubt on HJPEV's 'light' side qualities.

it's win-win, just like you know he likes it.