Some do that kind of breeding so that is plausible.
HOWEVER.. pointedly ask if the black roo is a lavender carrier.
A pure black over lavenders will give all solid blacks.
A black that carries lavender over lavenders will give half lavender HALF black chicks. It's not sex linked so there will be a mix of male/female lavenders or blacks.
On re-reading, if her breeding group has more than one rooster- say, one lavender one black rooster over a bunch of lavender hens.. if the black is a lavender carrier, then the answer is right as in theory both roosters are breeding all the hens, then more than half of the chicks should come out lavender with the chance of some blacks.
It gets a little trickier if the black is not a carrier and you have no real way to tell if the lavender roo is also breeding the hens or the black is either the dominant or the hens just prefer him.. in this case it's a gamble.