Short version: A friend that I've drifted apart from, recently left her long term boyfriend and now I'm going out with him. I'm worried about her reaction and also concerned I'm a rebound.

Long version:
A few years ago I was very good friend with this girl, we were in a lot of the same classes in college, had a lot in common and we ended up becoming housemates. Her boyfriend and another couple of friends lived with us as well. I got along great with her boyfriend and he was pretty cute. I had to remind myself a few times not to get too attached to him and to try and think of him as more of a brother figure. It didn't really work but I hid my feelings well. I don't think they ever knew.

Anyway time went on, we graduated, I moved out and we all drifted apart. I hadn't seen either of them properly in a couple of years and besides the odd conversation with her on facebook and twitter we didn't really stay in touch. They were still together, they both had good jobs, were talking marriage, mortgages and kids. They had their lives planned out. And then my friend left him for another guy six months ago.

Her now ex-boyfriend started talking to me a lot more after that, it started with liking my fb posts and tweets, eventually we were texting all day every day. Now we're dating and its looking like this could be heading towards serious.

I should be happy but I just have so many niggling concerns.
Even though I'm not super close with this girl anymore I still care about her a lot and I don't want her to be hurt or feel betrayed like I was biding my time till they broke up. We have enough mutual friends that she will find out eventually and some awkwardness is unavoidable, but is there a way of limiting the damage?

And there's another part of me that worries I'm just a rebound. Someone who is similar in appearance to his ex (same height, skin tone, hair colour and length, eye colour) with similar interests and enough history together that he can jump into a close relationship easily. If that is the case he may not even be aware that he's doing it. But he may wake up one day soon and realize I'm a poor substitute.
Is there a way of knowing if you're a rebound, any warning signs I should look out for?
Can a rebound ever become the real deal? Is it possible to make it work?

Sorry for the long post, but any advice is appreciated