I was in both your shoes and "Jack's" shoes at one time or another...and all I can say is...tread lightly.
Accusing her of cheating is a SURE-FIRE way of driving her away (and probably into his shorts).
I've both been the winner (comforting the girl) and the loser (accusing the girl) after such a confrontation.
Good way to play it: Be civil, but not overly friendly.
The in-jokes...VERY bad sign. Let her know it makes you feel distant from her.
If she isn't interested AT ALL in spending time with you (and ONLY you) when you two see each other...assume that the train has already left the station.
Also realize that she may already too far gone with this dude, and be prepared to cut your losses, if you don't wanna get played BADLY.
But if she isn't too far gone, I'll tell you the fastest way to snap her back to reality...
Get yourself a female "buddy" much like "Jack" is to her.
GREAT GOOGLY MOOGLY...but the sex is OUTTASIGHT when your gf/wife/whatever is out to rub it in some real (or imagined) competition's face.
BTW: The situation in college where I was "Jack" ended badly.
She went back to her boyfriend, after having sex with both of us, on and off, for a good six months.
She was roommates with my roommate's gf, which made certain things awkward.
She actually found me via the Internet about 6 months ago and wanted to meet...until she found out I was married

.
Her situation?
Married (not to her bf from back then) for 7 years...now divorced (about a year)...and in a relationship with someone at work (wonderful idea there).
I count myself VERY fortunate.
Of course, Mrs. Tallgeese still wanted to stomp her @ss, because, as always....
MRS. TALLGEESE = BADASS