Ih ave a bad situation what should I do?

So sorry for the short novel in advance, I want to make sure everyone has a full understanding so they can throw out their two cents… Anyway i've been dating this girl for a little over three months now and I'm leaving in four days for Marine Corps basic training (for those nit pickers out there looking at account history my dad was a marine and started this account and no longer uses it). She did know that I was in the DEP and leaving this month when we started dating and she was the one to actually get the ball rolling cause I was afraid to start any relationships. She already has her hotel reservation for my graduation along with my parents and a couple friends of ours booked so, and obviously there's no guarantee, but it would seem she plans on waiting for me when I come back in June. With that out of the way, here's the problem. We use to work together, I no longer work there and I don't plan on it ever again but another guy that the same restaurant keeps bothering her. At first, before we we're dating, it was all the time. She kept telling him no she wasn't interested and to leave her alone, but he was for lack of a better term "hooked" Now he texts her every once and a while trying to get a conversation out of her, and more recently he asked if she was still with me. I'm not the jealous type and I don't care hes talking to her but I care that it bothers her. There isn't anything we can do short of changing her number because they both have iphones so he can still imessage her if she blocks the number, and the police aren't going to be any help. So I want to make sure he doesn't start going over board with it when he finds out I'm away, but its not like i can just go beat the poo out of him either, it could effect my deployment and it isn't called for… Yet. So I was thinking of using the passive way and talking to his sister, who happens to know both me and my girlfirend but has stayed neutral through the whole thing. And I don't think I could blame her. I thought that if I said i didn't want to cause a fight by going directly to her bro, but ask her to ask him to just give the hell up everything would be alright. But that's why I'm here. Should I go directly to the source and cause a potential fight, or drag in a middle man and hope everything works out OK?

I think you should talk to his sister. If she doesn't seem like she will help then you should talk to him but don't start a fight. Try not to do anything that will get you in trouble. You should tell your girlfriend to completely ignore this guy, he sounds like a stalker. I hope everything works out for you. Best of luck with the marines.