I don't necessarily disagree but it's clear that OP still has some type of intimacy going on with the two considering the gf picked up pretty clear hints there at least was something going on in the past. I know platonic friendship can look differently for everyone but when you bring a new person into your life with that sort of info they deserve to know upfront and potential boundaries and questions/answers should have been put forth.

It was a trainwreck simply because they thought hiding pertinent info from the S/O was a good idea. I do hope the two can further talk things out, the friendship can be kept, and boundaries can be put in place to ensure everyone's happy and comfortable.