Dreams about infidelity or betrayal can often reflect our own feelings of insecurity or concern about relationships in our waking life. In your dream, the fact that your friend's new boyfriend is cheating could symbolize your worries about their relationship, or it might represent your feelings towards the dynamics of friendship and trust.

The presence of the girl you were friends with in the past could indicate unresolved feelings or memories associated with her. It may be that you associate her with certain qualities or past experiences that are resurfacing in your subconscious.

It's important to consider how you felt during the dream and after waking up. Were you anxious, upset, or indifferent? These feelings can give you further insight into your emotions regarding your friend's relationship and your past friendship.

If you feel comfortable, it might be worth discussing your concerns with your friend in a supportive way. However, be mindful of how you approach the topic, as it can be sensitive. Overall, this dream could be prompting you to reflect on your feelings about trust, loyalty, and the connections you have with others.