Does the Form 1099-R provided by the payer have box 2b Taxable amount not determined marked? If not, payer is reporting the amount in box 2a as the actual taxable amount and unless you have documentation to support the assertion that the box 2a amount is not correct you should not be using either the Simplified Method or the General Rule, you should be accepting the box 2a amount as correct.
If box 2b Taxable amount not determined is marked, see Taxation of Periodic Payments IRS Pub 575. If your mother made no after-tax payments into the plan, the entire amount is taxable. In most cases, the Simplified Method must be used for pensions when after-tax contributions were made and the taxable amount is not explicitly indicated: https://www.irs.gov/pub/irs-pdf/p575.pdf