Had a sheer cheesecloth dress once that split. I used the stuff that they iron on facings on blouses/ladies shirts (the bit behind the button and buttonhole turnings on the front). I bought the sheerest (lightest weight) one you can get, laid it behind the tear, taking care to line up the edges evenly, then lightly ironed the material (in your case the lace). Not sure what colour your dress is but shouldn't show through if you get the very fine iron-on fabric. Good luck.