Secretly dating my ex's friend, and now my ex wants me back?

My ex and I broke up in the beginning of February 2017 after two years of being in a relationship. After I found out he'd been lying to me about several major things during our relationship, things became extremely unstable. When he broke up with me, he told me I needed to start moving on and that his feelings for me were "all but gone." This broke my heart, but cemented the fact that I needed to get over him. We remained friends and occasionally talked, but we were officially done.
A few weeks after we broke up, I matched with my ex's friend (that I'd never met before) on Tinder. I told my ex over text that I matched with him (not in a spiteful way) and he got a kick out of it. Then he texted his friend, telling him he matched with his ex (since his friend had no idea who I was), and he seemed pretty embarrassed, but also told my ex that I was "ridiculously gorgeous." I came across his friend again on Tinder about a month later, and ended up matching with him again. We started messaging each other, and decided to go on a date about a week after texting. We've gone on several more dates since then (we haven't had sex yet, but have done almost everything else). We both agreed that we have a lot of chemistry, and things have started to get a lot more serious recently.
My ex texted me a few days ago, admitting that he still has feelings for me and wants to see me in person to talk. I don't have any remaining feelings for him, but I do still care about him as a person, which is why I'm hesitant to tell him I've been seeing his friend. I didn't start seeing his friend as a rebound, or to spite my ex. I have genuine feelings for him, and I'm finally happy after battling a serious bout of depression for months. Am I in the wrong for seeing his friend? What kind of reaction should I expect when I tell my ex about this? Furthermore, should he have the right to be angry that I've moved on after he straight up told me I should move on and that he had zero feelings for me?
Secretly dating my ex's friend, and now my ex wants me back?
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