She's talking rubbish.
She's misunderstanding how foremilk and hindmilk work and how babies gain weight.
Keeping a baby on one side only reduces the volume of milk the baby takes overall, and reduces the volume of milk made.
That might be a good thing - sometimes mothers overproduce and get all sorts of probs, and 'one sided feeding' can indeed help resolve this.
It might be a non-thing ie it doesn't matter if you use one side only at each feed. Some babies are happy and thrive on one side, and because most mothers are generous producers (given the right circumstances), they make sufficient milk anyway.
It's not necessarily a good thing to do this deliberately, though, as a few mothers do need the 'double sided' approach to make enough volume for the baby, and on top of that, leaving one side untouched for a while can lead to engorgement and worse.
It is volume of milk that keeps babies thriving.
The whole foremilk and hindmilk has bamboozled people into being worried about the 'balance' of foremilk and hindmilk, and they think (wrongly) that the baby can't gain weight unless he gets hindmilk.
This is what lies behind your HV's useless advice - she thinks the baby will gain more calories feeding one side only and he will not, I promise! His net intake of calories will reduce , as the volume of milk he takes in will lessen, and your supply will reduce - not what you want.
Foremilk is still good stuff, and packed full of calories - she's talking as if it is water!
Be guided by what your baby seems to want, and if he takes that second side after showing you he's had what he wants off the first side, you can continue giving him that second side.
And change your HV if you can - she sounds the worst sort....thinks she knows it all and yet she will damage breastfeeding.
www.kellymom/com has tons of stuff on all of this with references.