Lactose is a milk sugar, and after bouts of gastro it is very common for adults and children to have trouble breaking it down, and as your HV said it can take a few weeks until it can be tolerated again.
The thing is, there is lactose in breast milk, no matter if you are consuming dairy or not. There is lactose in all mammal's milks. So you going off of lactose is not going to remove lactose completely from your breastmilk.
From what I know, though, in the cases of temporary (secondary) lactose intolerance you are not supposed to stop breastfeeding...she will get better in time and although it can take a few weeks it also might just take a few days.
FWIW, if you did choose to remove lactose from your own diet, you would not need to remove all dairy, you could just switch to lactose free dairy instead. Cow's milk protein and lactose are completely different things, (although both are found in dairy) so there is no need to look at the proteins at all. However, if you suspect that the cause of her reflux and her diarrhoea is a milk allergy, that is the proteins, and yes they would need to be eliminated from your diet if you chose to keep breastfeeding.
HTH?