Boyfriends (were talking truthfully) wants to be friends?!?!?!

Long story short today I was on an app called House party with the guy I’m currently talking to. We’ve been talking for a month today. He’s never told me his exes name because we live in such a small world and he didn’t want drama so early. But I already knew her who his ex was prior to us talking through a friend. I didn’t know that his ex was a friend of mine on an app called Houseparty. (It’s like Skype but you can have more than 2 people at once and lock the so called rooms so people can’t join or see your active or who you are in a room with. Last night we forgot to lock the room so she joined the call for a split second and he freaked out because he didn’t tell me that was his ex and thought I didn’t know and that would be a very awkward encounter. He immediately locked the room (virtual room) so she couldn’t join once she left. I asked him if he knew her, he said “yes, but I haven’t talked to her for a minute”. And didn’t tell me that was his ex, even though I already knew. Today we were on the app talking, his ex joined again and he left and she tried to talk to me but I left. She was then waving at me to come join her room, so I did. She asked me why I always talking to him on the app and if I knew him. We began to converse for while.

We called on the app for the long time and she even asked me for my number and we called on the phone. She was telling me about their relationship and their time together. And she seemed as if she really missed him lowkey. But she was Nice and we had a lot in common(he even agrees) and we both really liked each other and forming a friendship I guess. I reluctantly told the guy I’m talking to everything she told me and how I thought she wanted him back. He didn’t feel the same way and wants to be with me and wants to let go of his past. However, my amazing soon to be boyfriend said it’s up to me to make a decision if I want to be friends with her and said he thinks “that shouldn’t of happened and he was trying to avoid us being friends as that has caused issues in his past relationships and said “I believe this will bring us(me and her being friends) problems that we don’t need. Now with my opinion being said it’s up to you what you do”. Their both 18 turning 19 and I’m 17. She took his virginity and they both never talked about why they broke up so they both have completely different ideas on why. Each time they talk again they end up getting back together according to her. I’m a naive and innocent girl. I see everyone’s intentions as positive and never the irrational. And as much as I wanna believe her intentions of befriending me are probably bad, apart of be believes she just wants to be friends and doesn’t mind. I feel guilty because they were dating for a whole year and had such a great connection even though I shouldn’t and have a lot of feelings for him. I get along with her so well and have heals in common and I wanna be friends and so does she but will this negatively effect my relationship down the track? Can I be her friend and keep her out of my relationship at all cost or cut her off completely like the guy I’m talking to suggest? also comment experiences both posit she negative with your partners ex!!!!