Hi, I use W&N's Finity range which I'm guessing is what your friend has? Yes, it is the artist range. I've not really had trouble with them mixing with Liquitex medium, but I only use the fluid matt medium. I wonder if the transparency issue is just with some colours or all of them? I ask because sometimes I actually use Liquitex paints for more solid, bright colour. Never thought about why though! I guess my only advice would be not to add medium to the paint, which will only make it less opaque.
Not much help, sorry.
But I'd guess a lot of people probably use Finity because W&N is so common.