My sister and friend bought stock together - the 1099B is in her SS - Can I split the gains between the two of them? Or does she have to file in her name only?

If the account is in your sisters' name only (that is, she and her friend worked out this joint purchase "on the side"), then she should report the full amount, and pay tax accordingly. Then presumably your sister and her friend work out between themselves how the profits get split and how much the friend should reimburse your sister for the tax bite.

If both people are named on the account - i.e. this is a joint account - then she can list a portion of the amount from the 1099-B and attach a statement exlaining that the proceeds are split with another person. The statement should include the friend's name, address, and social security number. The friend should attach a similar statement to her return, so the IRS can see that between the two of them the full amount of the sale accounted for.