spuds in a sack

A friend of mine wants to grow spuds in a sack, I was wondering if instead of using the expensive grow sacks if ordinary hessian sacks would do

They'd need to check whether light comes through the hessian - otherwise they'll end up with green toxic spuds!   But yes, empty large compost sacks with drainage holes in them would do fine.  Fill them half full initially, then roll up the sides and add more compost (like earthing up) as the plants grow

Just leave them to grow, assuming it's a 75L or bigger bag, there should be enough compost and depth to equate to growing in the soil.  Don't chop the tops off or the plant can't photosynthesise and bulk up the tubers any further etc

thank you i would have definitely got it wrong without your help

As said above - compost bags or rubble sacks will do a great job.
Hessian no good....water would go straight in & out and light would get in.
Last year I grew potatoes - all the same variety - 3 different ways.
In the ground in trenches, the traditional way....in buckets in compost....and in compost bags in compost(75L)
Those in the compost bags were by far the best.
The ones in the ground were dirty (yes, potatoes usually are!) but also had a lot of blemishes/scab/infestations/spade damage....those in buckets I was forever picking slugs off, although they were better quality than those ground-sown.
The winner by far was those grown in compost bags. A huge crop of beautiful clean potatoes, needing nothing more than a wipe prior to cooking....and not a mark or blemish on any of them. And no pest in sight. No spade damage. None overlooked & left to grow next year in a new bed among the cabbages or corn!
I'm absolutely no expert, but of the 3 ways I experimented with last year - all the same variety, sown at the same time - the bags proved way superior to the rest in the end result
I hope your friend has great success too.
Tess