Without her giving it to you voluntariliy you have little recourse. Identify theft is difficult and illegal. Have you got any old documents that you both used to buy something on credit, or rent a house or apartment. Most people have a junk drawer full of old invoices, contracts, paid bills, leases, instruction manuals and other outdated documents. scour those documents if you have any and you just might come up with her SSN.
You can also, for a fee use
Intelius and get her address and maybe a phone number, although I would write first before I call out of the blue and explain your situation and see if she will cut you a break and help you get the Pokerstars acct switchted to your bank acct, which is actually good banking practice on her part..
Why does she want Pokerstars to have her even old banking info on their database (good way to help convince her to help you).
Mainly you have to come across as an honest and intelligent person just trying to straighten out a difficult problem that requires her help as a friend. One who would be very appreciative to.
Most people with half a brain will not give you their SSN but she if you put it to her right (don't try to get back together with her during this process if you find her) and be all business and let her know you just want to get your Poker acct straightened out, she just might take the time to contact Pokerstars or do whatever it is you need her to do to get the account usable again.
Odds she will just give you her SSN are not good.
Best of luck, wilheim