Concern or control?

Question... In a relationship at what point does a person's constant "concern" about your whereabouts turn into an unhealthy control? It is understood that when you get into a relationship there are certain courtesies you extend to your new partner. You let them know where you are going, you call on regular basis, things like that. But is there a line? I have an associate that has been with her guy for a while, he gets upset if she doesn't call as soon as she gets in the house or if he sends a text and she doesn't respond right away. If she goes out without him a few too many times he will get upset and tell her she's "acting like she's single". He likes to know what she's doing all the time and be involved in all her decision making. Now the two don't live together at the moment but they plan on eventually getting married. She is already beginning to feel smothered by the tight leash he keeps her on. I personally believe the grip will only get tighter as they continue their relationship. But what does she do. Is this just her man showing loving concern for her or is he a bit too controlling. Should she be endeared or concerned?