Do not sign that form for your husband. Instead, you should be filing an application right away for a court to compel your husband to produce his income information and to sign that form.
DISCLAIMER This answer is provided for educational purposes only. By using or participating in this site you agree and understand that there is no attorney client privilege between you and the attorney responding. This site cannot be used as a substitute for competent legal advice from a licensed attorney that practices law in the State where this offense is charged; and, who has experience in the area of law you are asking questions about and with whom you would have an attorney client relationship. The law changes frequently and varies from jurisdiction to jurisdiction. The information and materials provided are general in nature, and may not apply to a specific factual or legal circumstance described in the question, or in the State where this charge is filed.
It's not a clear answer, except for one aspect -- do NOT sign the form "for your husband" (electronically or otherwise).
First suggestion - call the FASFA folks and explain the situation; that you're separated and he won't provide his info or sign. See what they suggest.
Second suggestion - get it before a judge, seeking an order to compel you ex to cooperate with the process (even if that means his giving the info just to FASFA and not you, if he's claiming a privacy issue -- that shouldn't hold water in the long run as you're entitled to that info in the divorce process, but it would address the issue for now without endangering your child's receipt of financial aid). I'd also ask the judge to permit you to sign "as attorney in fact" (as if you were him), but don't do so without a judge's permission.
DO NOT SIGN FOR HIM ON THE FORM. Aside from the issues that could be raised with fraud or forgery, there could also be implications as to what your husband would be required to pay of any balance that is not covered by the FAFSA. Your husband could try to claim he was not consulted about college selections or financial aid as a way to try to avoid paying.
You should file an application with th ecourt immediately and ask the court to compel him to sign the form and to allow you to sign if he refuses. As for the financial information for 2010, ultimatley he is going to be required to produce this information in discovery or with his Case Information Statement so you should be able to update the information before any final deadlines for FAFSA.