The stock cog is 16t so yes, you'll need to add at least one full link. I had trouble finding the sweet spot when I switched to 48/18. I originally added two links but when my chain tension was optimal, my axle was at the very end of my dropouts and I wasn't digging it. I removed a link and it brought my wheel up closer than I was used to and it takes some clever workings to remove my rear wheel but I'd rather deal with that than have my rear wheel look like it's about ready to fall off.
It sounds like her bike has a 3/32" chain and your freewheel is 1/8".