Reference: Majmoo’ Fataawa wa Rasaa.il ‘Abdul-‘Azeez ibn Baaz – Volume 16, Page 405
Question: My Mother performed the Hajj seven times, so is it permissible for me myself to perform [another] Hajj on her behalf?
Response: Yes, it is permissible for you to perform [another] Hajj on her behalf – an eighth Hajj or more – and this is from kindness [and honouring] towards her, and there is great reward in that for you if you have [already] performed the [obligatory] Hajj for yourself.