Interesting topic, especially since he doesn't know what she's done..
I'd say yes, he should be able to opt out legally. Should he? No, I don't really think so, but that's on a moral basis. Since it is his child, and theoretically, shit happens. (as you do have to understand that even with precautionary measures, pregnancy can still occur. Plus, he's unaware that she tampered with the condom) Thus, he should know that if he really loves her and cares for what is his offspring, he wouldn't leave a kid fatherless and pack up.
If, he did know that she tampered with things, absolutely he should say fuck it, and leave.