she should tell him. it's true that he shouldn't have told her he was clean if he wasn't, but then, in my opinion, she ought not to have slept with him without a condom. the other possibility is that he was tested and given the all-clear but had since developed an STI - i think i'm right in saying that there are a couple of condition which, although sexually transmitted, can be developed without the person having had sex with an infected individual.
either way, he has a right to know and - what's more - by not telling him, your friend's actions could compromise the health of other girls who sleep with him in the future.