What you are saying doesn't make much sense.. It is soluble in HF but dissolves if NaF is present?
... it seems like they are almost contradicting each other the way you worded it...
My guess for what it is you are asking is that the reason why AlF3 is more soluble in NaF than in HF is because the Na+ and the F- fully dissociate, while HF is a weak acid and does not?
or vice versa.. not sure haha