Thanks for clearing that up for me. For some reason I thought I heard her say that the boys would be identical...and how does she know it's a boy??
Theoretically, one could:
(1) select the "sex" of the sperm. This is not all that accurate, but "female" sperm are heavier (and some other different characteristics), being that they have an "X" chromosome, instead of a "Y" and can be sort with some accuracy using a cetrifuge and other methods or
(2) an embryologist can take one cell of the embryo (a blastocyst at the time --- like 32 cells) and do a DNA test on it. This test is very accurrate, but delicate procedurally AND ethically. Not many labs could do it, I would suspect.
or
(3) they could have discovered that the husband has a slightly defective "X" chromosome, which would make all female babies non-viable. With this knowledge, they would know that any embryo that reached the blastocysts stage was almost certainly a boy. (This is actually fairly likely, as it is a cause of a lot of male-based infertility --- half the sperm are non-viable, so your chances of pregnancy drop accordingly.
or
(4) she's just guessing or using "he" in a sex-nuetral manner.