translocation in general is when a part of one chromosome breaks off and attaches to another another, ie. your chromosome 18 breaks one of its extremities and it attaches to your chromosome 13.
when it's balanced, it means that also another translocation occurs: a piece of your chromosome 13 breaks off as well and glues to the chromosome 18 that was already broken.
so in the end you get to keep all your genetic material. this happens usually as she said at an embryo stage, and since all the genes are there, the embryo develops normally and then the person carries it without knowing (unless the break occurred in the middle of an essential gene and fucks up its function).
the problems occur only when it comes to having children, as in her OH's case, when the sperm is made you have 4 options: a sperm has both translocated chromosomes, or translocated 18 and normal 13, or translocated 13 and normal 18 or both normal 18 and 13.
so this is why 75% chance of miscarriage: when sperm fertilizes a normal egg, male and female chromosomes pair between them and are supposed to mix (recombinate) to generate a new mixture of genes - your baby. the thing is, when all chromosomes are normal, pairing goes well and you get all the genes present in two copies, one from the mom and one from the dad.
but when you get these translocated ones... well the mixing part doesn't go well as ie. when two chromosomes 13 are joined to mix, one normal from the mom and the other from the dad with an extra piece of chromosome 18 attached ...
many errors occur, parts of genes on chromosome 13 are lost and the genes from chromosome 18 are overdosed (as they end up being present not only on both chromosomes 18 that already come from mom and dad alone, but plus they also end up being included in this weird chromosome 13 mixture now).
so the embryonic development can't proceed anymore beyond certain stages... usually 6 to 8 weeks when main organs and systems of organs start to define.
hope this was clear
i hope you can see the pic, i tried to post/upload but it didn't work
https://upload.wikimedia.org/wikipedia/commons/c/cb/Translocation-4-20.png