Can’t remember my firmware password

If you can't remember your firmware password, schedule an in-person service appointment with an Apple Store or Apple Authorized Service Provider. Bring your Mac to the appointment, and bring your original receipt or invoice as proof of purchase.

From:

Set a firmware password on your Mac - Apple Support

Then get the receipt from her, along with a letter verifying the you purchased it from her along with a copy of her ID, or just have her take it in.

There's not going to be an easy way around this, the whole point of a firmware password is increased security, so if it was a simple matter to reset it, that would defeat its purpose.