Granted marriage visa Is an earlier visit possible?

[Loi001]

Hi I'm asking this question for my mum(Malaysian)
She and her partner (British) have been granted a Marriage visa to get married in the uk. The visa has been granted for march to sept
However she has now decided she really would like to come earlier to celebrate my daughters birthday (we are settled in the uk now) which is early feb.
My question is
Malaysians do not need a visa to come to the uk but if she enters will the tourist visa stamp void her marriage visa ?
Can she leave before march and reenter the country on or after the visa date?
With the marriage visa does she haveto travel from Malaysia where the visa was provided?

Thanks very much for any help or advice!
My 5 year old is getting very excited about her grandma coming for her birthday !