Summarize this article:

A controlling person often discourages or forbids their partner from spending time with others. They may speak badly of their partner's friends and family or act jealous when their partner sees other people. If your friend or family member has become less and less available after getting together with their partner, it could be a warning sign that their partner is trying to isolate them. If the person tells you their partner doesn't approve of their friends or social life, it could be another red flag. Controlling people try to gain the upper hand over their partner by breaking down their self-esteem. One way they do this is by criticizing their partner's appearance, personality, or abilities. Criticism may be overt, or it may take the form of backhanded compliments or hurtful "jokes." Notice if the person's partner says things like "You'd look so great if you lost some weight" or "Why are you going back to school? You were no good at school before." Does the person's partner get them to do things they normally wouldn't do? Using guilt, threats, or pressure to control a partner's behavior is a common tactic in unhealthy relationships. If your friend or family member has been acting out of character lately, consider whether their partner might have something to do with it. Statements like "If you ever left me, I'd probably kill myself" or "I do all these things for you, and then you repay me by making your own plans and leaving me alone" are giveaways of a manipulative relationship. Be aware that your friend's safety or even life might be threatened, and they could be unwilling to disclose that. The harder it is for them to make contact with you, the more serious the situation may be. Controlling partners always want to know what the other person is doing and with whom. If your friend or family member has to call their partner frequently to check in, or if they have to get their partner's permission to make plans with other people, the relationship is probably unhealthy. Another major red flag is if the person's partner reads their text and email conversations.

Summary:
Notice patterns of isolation. Be alert for excessive criticism. Keep an eye out for manipulative behavior. Notice if your friend's partner is suspicious or nosy.