Hi Cruzeen
As a son of the deceased, your partner has a legal right to see/obtain a certified copy of the Will.
How long ago did you partners mum pass away?
The first step to take is to write to the Executor and make them aware of their legal obligations to the family of the deceased (which includes your partner). I have found that this is often required, as the Executors often bully relatives who are unaware of their legal rights.
Next, or alternatively, if the estate has gone to probate, a copy of the Grant of Probate, which contains a copy of the Will can be obtained directly from the Supreme Court.
Lastly, an application can be made to the Court, to order the Executor to comply, and I would suggest that the Executor would be responsible for the legal costs to do so...
Kind regards