friction shifting gears/wheelset replacement

working on sorting out a wheel upgrade for my gf's bike.

currently have a 6 speed freewheel style cassette in decent condition, but the hubs are shot, the spokes look old and the rims don't have a decent breaking surface, don't inspire confidence...

anyway it uses friction shifting, can I get lower count free hub style cassette and wheel and expect it to work just as well?

Maybe. Trial and error is the way to go.

You can have as many gears as your RD travel will allow. You can use 7/8 speed chains and cassettes but might not get access to all the cogs. If your RD won't travel the full cassette then set the min/max travel to the gears you want to use. Cheaper Shimano cassettes come as one fixed unit, more expensive ones have individual cogs and spacers so you could pick the cogs you want and use spacers for the dead space.
Check your hub spacing though? But if it's steel you can go a size either way no probs. Aluminium is more brittle though.

When I needed a new rear wheel for my old 126mm rear spaced Peugeot I bought a second hand Shimano 600 hub / Mavic MA40 wheel on eBay (or the Classifieds here, can't remember), which worked great for a while, and can be found very cheap.

Later got another 600 hub and a NOS cassette and had it built with DT Swiss spokes onto a Mavic Open Pro for something more durable. No problems so far after over a year using it, and the rim doesn't look out of place on the old frame. Matches my front MA40.

I use a real mix on my winter beater. Namely 7 speed shifters on friction setting, an eight speed rear mech and a nine speed cassette! It all works really well. Most bike shops will be able to widen your frame dropouts to 130mm.
This means you can use modern wheels. This means also that you can obtain some real bargains.