This is a good question and one that I've held off answering because I wanted to re-watch the episode first.

Gloria could have very well been charged as an accessory later on as Lew gave his testimony. He did tell Danno in the car all he could think about was arresting Oliver after "she called". Of course, this wasn't the case because Lew intended on killing Oliver from the word go. Perhaps because of that fact, Gloria wouldn't be held as an accessory because she didn't know what Lew's true intent was.

I would also say, Lew deserves an Academy Award for all his acting. He's shell-shocked when he's told his wife was having an affair not to mention when he barges in at the beginning of the show playing the aggrieved husband. He's a cad - checking out his handiwork as the lab gets done on the inside of the house. To me this makes Lew Morgan one of the creepiest of Five-O villains because he knew *exactly* what he was doing when. I can only imagine what kind of a mess the case would have been if McGarrett agreed to let him help.