Most cysts, even complex ones, are benign so try to relax until you have more information. The reason that they usually take out complex cysts is because if cancer is present it usually looks like a complex cyst or a solid tumour. HOWEVER, the large percentage of complex cysts are completely benign.
Keep in mind that even if your daughter in law's CA-125 is elevated this DOES NOT necessarily mean that she has cancer. This test is notoriously unreliable and can sometimes be elevated due to benign reasons such as being on your period or endometriosis. The way the cyst looks on the ultrasound is often the best indicator prior to surgery and radiologists are quite good at seeing what is cancer and what isn't. Needless to say, the cyst will have to come out in order to be sure.
My sister in law was terrified when they discovered she had a 12 cm septated complex cyst that took up BOTH of her ovaries. Her CA-125 was elevated and she went to see a gynecological oncologist who performed the surgery (under these circumstances you really need an oncologist in case it DOES end up being cancer and more aggressive surgery is necessary). After all of this anxiety it turned out to be a perfectly benign growth although she did loose one ovary due to the cyst.
My cousin has had cysts all of her life and when she was only 20 years of age one of the cysts twisted her ovary and cut off the blood supply so that she ended up loosing an ovary. Despite this, she had two beautiful children. She recently began to develop complex cysts on her other ovary and decided to have the other ovary removed in order to prevent having to go in and out of surgery (she had already had other cysts removed from this ovary). The cysts were benign and now she is in surgical menopause at the age of 32.
I myself have an 8 cm fluid filled cyst on my left ovary and was worried sick that it may be cancer. They found it last September and I have kept an eye on it ever since. I did have the CA-125 done and mine came out at 7 (anything under 35 is considered normal). If my cyst begins to grow or change in any way, I will have to have a laparoscopy to remove it and possibly my left ovary as well. I am not happy at the prospect but I have accepted the situation ( more or less
Ovarian cysts are EXTREMELY common occurences, even complex ones so don't worry unless you have something to worry about (and you probably don't).
All the best and come back and tell us how your daughter in law is doing.