What if you refuse to train for combat medic in army my granddaughter completed boot camp and has now decided to refuse to train for combat medic and just wants to come home to boyfriend?

She's training as a "Healthcare Specialist", not a Combat Medic. She can refuse to train if she wishes. Of course, she'll have to face the music. If she fails to report in, there'll be administrative action taken against her, and she'll be discharged under dishonourable conditions. It may not seem like a big deal to her, but it will bring quite a bit of bias against her in the civil sector, and will impact her life in a very negative way. Not to try coming across as rude, but it seems to me like she didn't think her decision through before she enlisted. Which, it happens to people. But, her choices at this point are either fulfill the obligation she signed up for, or reap the whirlwind that comes from refusing to do it. Lots of people in this world are in places or situations they don't want to be in, but, of course, they can't just shirk it and forget about it. Why would you think the military was any different?
Furthermore, when you enlist into the military, you sign a legally binding contract. It's not like working at your local 7-11, when you can quit whenever you please.
If she wants to ruin her life for the sake of a boyfriend she's probably not going to be with for the rest of her life, then that's her prerogative. But an attempt should be made to inform her of the potential consequences of her actions. The fact that she seems unaware of them now indicates to me that she hasn't taken any of this into consideration, and it's likely she won't want to hear it now. She's been given enough rope to hang herself with at this point. As she is legally an adult at this point, she's the one who decides whether or not she lets herself get hung up.

Good advice above. Just need to add she will be separated with a very unfavorable characterization of service, likely an under other than honorable conditions (OTH) characterization. I work on the Navy Discharge Review Board (NDRB) and have seen thousands of cases from young, dumb sailors and Marines who have literally ruined any chance of decent employment for the rest of their lives. If she thinks she'll get her discharge upgraded, better think again...the Army DRB doesn't play, and she will likely find it VERY difficult to recieve an upgrade. Tell her to suck it up, and be the best soldier she can be until her enlistment is over. She'll be glad she did.