My Girlfriend is over 500 pounds and I'm worried about her! What do I do?

It's great that you are so supportive and loving, making it clear that that you love her and you are with her and supportive of her no matter what. Nagging and threatening is what most people do and it doesn't help. It just makes matters worse. 

If you have not made it clear how worried you are that you will lose her, do it now. Ask her to go to counseling with you. Look for a counselor who has expertise with couple's or family therapy and also with eating disorders. Use the "Find a Therapist" function at Psychology Today.

She will probably resist at first. Be patient, don't nag, and persist gently with the request. There will come the day she says yes. Make the call, set the appointment, and I'm sure it will help get you going in the right direction.