I ask you. What kind of an answer is that? Why even bother? Why hedge with "probably", since it is absolutely comprehensive in its lack of helpful advice?
Arms and shoulders of lace dresses are especially prone to tears. The wearer may have been ever so careful getting in at the beginning of the evening, but toward the end, getting out, other priorities may have resulted in a rushed exit, and consequent rips.
Vintage buyers should be aware that tears such as these are easy to miss, especially when buying on the basis of scans. Always ask specifically about flaws in lacework before purchase. The main issue in repairing lace fabric is seeing the damage accurately. You need to put something underneath it to heighten the contrast--a white background for black lace, or black with white. I made an arm-diameter tube with thin dry-cleaners cardboard and tape, then covered it with a couple of my husband's white tube socks (he doesn't know this).
The tube, slipped into the sleeve, made a workable mount to pin the lace to. Then, using fine black thread, I bridged the gaps in the net with a series of loose stitches roughly imitating a spider's web (rather than a straight line of them, which would have resulted in a visible scar). The mend isn't perfect but it's good enough to minimize the large gaps; plus reinforcing the lace in this way will keep it from tearing further.