In regards to references - employers are only required to confirm dates you were employed. They can't give you a negative reference because they contacted you after your employment ceased to do some work... and you 'shock' set them straight about the fact you longer work there and that ex-colleagues is askong you for help/to do some work.
Secondly - excolleague shouldn't have contacted you. hR or Management would have if it was an official request. She has been sly here. If it were my business, I would want to know if staff are conducting themselves this way so that I can address it.
OP -as a Pp said, the info or help you provide could very much be out of date or no longer correct. If you do help without being given the correct direction or updated info from the employer, your help could turn out to be a costly hindrance.
I'd still email CF colleague telling her you don't work.
And I'd send a separate email to the boss to say the same. Keep the boss on side by forwarding the email. Thanking them for thinking of you but unfortunately you are no longer employed. Ask the boss to call you to discuss either payment for the work, re-employment or to confirm you are not required.
As I said before CF has been very cheeky and probably gone behind their backs. I wouldn't answer to her at all, other than to say that you no longer work there. x