Since there is insufficient information in the original post,I find it difficult to respond to the case mentioned. I do agree it takes two to tango and protection is a mutual issue just as much as mutually consentual sex.
I must consider the sex involved before deciding on this one. Let's assume the infected person was the recipient of unprotected sex. If that were the case, why is there a need to disclose when he is the person at greater risk. Then I defer to points made earlier; it is a mutual decision and depending on what actually occured it seems difficult for me to presume to know who was actually at risk. I also think it demonstrates a lack of character when an individual does not share information that the partner may want to know. If no one asked and he never told, I say two people are engaged in risky behavior. But that's just my two cents too!
When you stumble, make it part of the dance.