I have a friend who chews with his mouth open and talks at the same time. His wife and in-laws tried to stop that behavior and were happy when I finally chimed in that it was gross to look at him. He took the hint and has been doing very well lately. He does need a reminder once in a while.
Choosing the wrong fork is one thing (I've done that a time or three), but asking that he chew with his mouth closed is a reasonable request and will make him a more pleasant dinner companion.
She should put her foot down with her four close friends and interrupt whenever they start to put him down. She shouldn't have to ask them more than once, but should give them another chance. I would advise her never to share any boy friend problems with these friends because they sound like the kind of people who will throw them up to her later.

My mother used to nag us about our table manners as kids but, for some odd reason, as she got older she developed the same bad habit under discussion, of talking with her mouth full of food.

After numerous suggestions (which just angered her), I finally started picking up my plate and taking it into the next room when she would do that. Since she didn't like to eat alone that really drove the point home and she started being more considerate.

It's not fair for someone to ruin everyone else's appetite over a habit they could easily change with just a little self awareness.

It sounds like the issue really isn't a racial one at all, but simply one of bad manners. It also appears that the boyfriend picked up these manners at home. I agree with BBs advice completely and would try to get the guy to meet his girlfriend halfway on the table manners thing - him, no chewing with mouth open (or whatever), her, no allowing the friends to poke fun.