I think I'm going to tell my boyfriend that I'm find with him talking to his ex.
Is there a reason why you want him to talk to his ex?
Because I think she's a nice girl and I don't see anything wrong with him talking to her.
If you and your boyfriend are in a good relationship and trust each other, you're smart to encourage him to have more friends. being friends  with exes can lead to healthy and long lasting friendships for all involved as long as there is trust.
I agree, though I don't want them to get too close.
It's awesome that you and your boyfriend have a mutual trust and there's nothing inherently wrong with being friendly with ex's. So long as your boyfriend is a stand up guy, I'm sure he'll set boundaries and you'll have nothing to worry about when it comes to him talking to his ex.