You can still claim the 1095-A. If your son is on the form, and you claim him, then you can include it. When she input the information on her return, she probably claimed 100% of all the amounts listed. That would leave you with zero percent of the amounts.
You can indicate that you shared the policy and prorate. You can decide the proration amounts between you but the pro-rations must add up to 100%. What percentage of the following policy amounts do you want to claim on your tax return? See "How do I figure out these percentages?" when you get to that screen.
Child/Children on another tax return
If you share a policy with someone who is claimed on another tax return (usually a child), you can take the number of people on your tax return who are covered on that plan and divide it by the total number of people on the plan. This would be your percentage of premiums, SLCSP and advanced payment of the premium tax credit, unless you agreed on a different number beforehand. (This would have applied to her.)
