Bettas are fairly adaptable fish. While it is recommended to keep them in water that is soft/neutral, a pH of 7.8 is fine. It is better to leave the pH as it is, then to try and aim for the 'perfect' value.
One word of warning, watch the rock you are using, isn't one to push your water hardness and pH up. Some rocks such as limestone, will, so it is something to be mindful of.