Ask you homie to that sh#t out loud: "My chick is going on a 2 week trip with a dude who used to smash her out on a regular basis." Then ask him how that sounds.
Well, at this point, I don't think asking her to cancel would suffice since the trip was planned before they were together. HOWEVER, if they were dating 4 MONTHS before they actually made it official, then when exactly did she plan this trip? A year ago? Six months ago? Sounds fishy to me.
Not to mention, why did it take 4 MONTHS for them to get together? Maybe I'm trippin, but every chick that ended up being my girlfriend wanted to tie me down way quicker than 4 MONTHS. If anything, a chick knows pretty quick whether they feeling you really hard or not...and it don't take them 4 months. Sounds like your dude may have been courting this chick. And I hope he knows that while they were "unofficial" that she was "officially" getting banged out by dude.
Your homie needs to make some plans himself to make sure that a) he's not sitting around thinking about her the whole time b) that she's getting just as jealous if not more worrying about what he's doing. Oh, and he needs to make sure that she knows full well of his plans to have "fun" while she's away.
Lastly, he needs to let that phone game get extra janky during that two week stretch. I'm talking about "no answering her calls after 6pm" type janky. I'm talking about her sitting in her room wondering "why aint this n#gga picking up his phone...he better not be chillin with some chick" type janky. I mean "I can't wait to get back home so I can interrogate this dude playing me to the side" type janky.
Its never a good look for a dude to be jealous with chick...that's feminine. BUT its always good for your chick to be wondering what you doing when not with her.
Peace