There are two formulae she needs to consider. There are limits to the amount she can contribute to a pension scheme in order to qualify for tax relief, which are determined by her age. If she's in her 30s, the limit is 20% of net relevant earnings, if she's in her 40s it's 25% etc. This figure includes any contributions she may already be making to the main scheme.
The second is an overfunding calculation to make sure she's not aiming for a pension that's in excess of Revenue limits. In a simplified form, maximum permissible pension at retirement under an Occupational Pension Scheme is 2/3 final salary with allowances for increases in pensions during payment and spouses pensions. She needs to make sure that the combination of her main scheme benefits and additional contributions don't risk exceeding these limits.
Finally, unless she's got private income, any private contributions she makes must be either Additional Voluntary Contributions to the main scheme (AVCs) or an AVC PRSA.
It's a complex area and she should take professional advice.