You & your husband are also at risk. For example, since your boyfriend is not communicating about afairs or sex with his girlfriend, who is to say that she is not also 'cheating' or practicing non-monogamy without your boyfriends knowledge. I find this a possibility since you say "she knows" but it is still not discussed in the open. This potentially raises the std risk for everyone involved since there is basically no communication. Not to mention the fast-coming fall-out drama I forsee if something doesn't happen.
As stated by others, this also brings to light several psychological issues your boyfriend has with the idea of polyamory. He see's it as a dirty thing. This is definitely worthy of your discussion because it relates directly to you. You can encourage his & her perception of ethical non-monogamy to change to something positive, encourage honesty, and make choices for yourself based on agreements/boundaries that you & your husband agree on.