Since you're newly qualified, it's good extra bit of practice for you. As everyone else has said, just explain there may be a slight color difference as you may not be using the same range of products as their previous tech, and proceed if she's happy with that. In my experience, it's best to just file it down as thin as you can before putting your product over it, especially if they're coming from NSS because the natural nails will most likely be too thin and flimsy to support a new set if you soak the whole thing off. Also, I hope you're charging accordingly because at this point, it's more of a rebalance than an infill.