There are many aspects to this kind of situation.
How long have you and bf been together? You don't know him well enough to trust him? You're afraid this girl (an ex of his) will bring drama? Has he told you negative things about her from the past?
Seems he feels he has unfinished business with her? You could just love him and stand by and let him work it out. There's a chance it won't last long between him and her.
When I was first with my present gf, we were poly from the start. I was just out of a 30 year mono marriage though and still felt I needed to explore many people. I know she didn't think all of the guys I dated were really stellar, but they brought experiences to my life I needed to have. As long as I was physically safe, miss pixi stood by me and learned more about me from what I was needing to learn about my heart, my sex and kink interests, my freedom from monogamy, etc., with these men. For my part, I spent loads of time with her (while continuing to see others), dating, romancing, sexing, cuddling, cooking, just enjoying the hell out of each other. Heck, I still had NRE for her while seeing the other people, so it wasn't a chore to be with her!
Since you say you and your bf do not have rules, a veto is out of the question. But you still need reassurance you and he are OK while he dates this ex of his and goes where his heart is taking him.
So... yeah. What Gala Girl says. Ask for what you need to feel emotionally safe. Other than "dump that bitch."
Love withers under constraint; its very essence is liberty. It is compatible neither with envy, jealousy or fear. It is there most pure, perfect and unlimited when its votaries live in confidence, equality and unreserve. -- Shelley
me: Mags, 59, living with:
miss pixi, 37