I'll add a couple of more perspectives on this.

Some people will get a new gf or bf before breaking up with their current one, because maybe they really are done with their current gf/bf, but they prefer to set up a new relationship before leaving the old one.

I've never done this, but I've had the women in my life do this to me, from both angles.

Now the thing is, it's not necessarily a terrible thing from your perspective. Is it amoral to pull a Scott Pilgrim and start dating Ramona before breaking it off with Knives? That's for each person to decide, but it's a grey area, if you've ever watched Scott Pilgirm Versus The World. And if you haven't, you should, it's a freaking awesome movie with a ton of video game references.

HOWEVER, the one thing that happens in all of those situations, is that the other person does not know about the past relationship. Ramona doesn't know that Knives is with Scott when they meet. The girl I was dating, I didn't know that she was still living with her bf when I started dating her. And the girl I was in love with that met another guy, she didn't tell him about me, either. This is a normal protective mechanism for this type of dating style.

Regarding the situation as described above, I think the real red flag is that the boy in this case has told you he's engaged, and talks about his ex with you. Anyone I've ever known that was truly unhappy and "done" with a relationship, they really don't like talking about their ex (or soon to be ex). They're over it. And the fact that he's talking about making it work with her?

I know it might sound odd, but it's actually better if someone starts dating you, and has a significant other, if they hide that from you. I've actually seen quite a few relationships start in the Scott Pilgrim type way, and have them work out to be really great. The thing is, they do what Scott did - they dumped the ex and walked off into the moonlight with their new love, feeling sympathy for their ex but no real emotions for them. The fact that this guy hasn't already wiped the encounter with his current ex to re-start the encounter with the letter-writer is a very, very bad sign, and I would run, not walk, the hell away from him.