I would normally be firmly in the “men are allowed to have female friends” camp - I have male friends, my boyfriend has female friends, so did my exh before him (admittedly, he did end up starting an affair with one of them, but he is still with her now - that’s more a case of them being made for each other while he and I had a basically shit relationship - I don’t in any sense regret “allowing” their friendship when we were together.....)
However, this one is odd because it is not a friendship of long-standing (based on them being old childhood friends or something), nor is it a friendship that developed normally and organically through work or a shared hobby, as most genuine friendships do. I too would feel uncomfortable that he only met her once, and then out of that they both developed a maintained this close friendship - does that make sense?
As for the regretting they lost touch thing, again I think that would be fine if she had been his old childhood friend or something - but this just seems too loaded with emotions, given the circumstances....
Not sure what you can do tho, OP, because even tho your misgivings may be understandable, demanding he drop the friendship would still be (there are no two ways about it) a controlling act, and not one I, for one, would tolerate from a partner in any circumstances.
I would, however, speak to him as honestly as you can - explain your insecurities about it - that you are not accusing him of infidelity or trying to control him, but that it does trigger your insecurities and you can’t help that. I have done that before in similar circumstances, and found that discussing it all openly has helped......