Should I let his girlfriend know he is cheating?

No matter how one thinks about this situation from multiple perspectives, the answer to this question should always be... YES! You should definitely go ahead and tell his girlfriend about the truth (that he cheated on her).

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Regarding this, is my ex new relationship serious?

#1. After a long, serious relationship, you and your partner break up. A couple of weeks later, your ex starts seeing someone they just met. This is an obvious sign that it's a rebound. Your ex is just trying to fill the hole in their life created by your absence. Then, how do you know if your ex is happy with someone else? Noticing Changes in Communication. Notice if they ignore your phone calls. When you call your ex, notice whether they answer the phone or send you to voicemail. If they are regularly ignoring your phone calls (especially if this is a new behavior), this could be sign they've fallen for someone else.

Is it still a rebound after 6 months?

Six months is not a long time by any means, but it is still a good chunk of time spent with the other person. After a breakup, the dumper is also angry, sad and is missing the other person. So much in fact, that they sometimes find someone new in the first few weeks after a breakup. Thereof, what are the red flags in a relationship? Red flags in a relationship include excessive jealousy and frequent lying. You should also be wary of a partner who frequently criticizes you or puts you down. Another major red flag is an unwillingness to compromise - relationships shouldn't be one-sided. Visit Insider's Health Reference library for more advice.

Also, how do you know if he still loves his ex?

He is happy when he sees his E. It's the best indication that he is still harboring his ex if he's warm, kind, flirty, friendly, or feels really happy to see her. But, if you feel that he is really too happy to see his ex if they meet him by chance then it's a good chance that he still loves his ex.