The other guy, Andrew Sandler, is not entirely unlike ad-man Deutsch, in that he inherited his business from his father. Apparently he grew suspicious of wife Lisa, hired a private eye and ended up with a picture of the two kissing.
All this is now known because an anonymous source fed the info to Six, and neither Sandler (pictured) or his lawyer are bothering to deny anything, saying only "we're trying to work this out amicably." Good luck with that, especially with two kids involved.
Narcissist though he may be, Deutsch is unmarried, raising the question of why this whole mess was leaked. It seems most likely that Sandler wanted to embarrass Deutsch (though he declined to comment to Six, officially) and hurt his luster in the ad world and his chances of hosting a TV show again. It's hard to imagine how anyone else (like Deutsch) would know when the PI was hired, for example.
But Deutsch doesn't seem like the type to show much embarrassment over anything. When we wrote about him in May, two commenters mentioned his prowess in the sack. It's entirely possible he was proud of his conquest. Hopefully the bad publicity will instill some reticence about being in the spotlight. Shame is probably too much to ask for.