Hi, How many of her nails are really long? if it is just a couple then I would just file them down....if its more than 5 then the overlay would probably be best and tip the others....
IF and I mean IF you are confident that you can make them all look the same !! If you think that the client or anyone that sees these nails will be able to spot the ones that are tipped because they look different to the overlays ....then do a full set of tips and overlay...keep them all the same..until you get more experienced with overlays....
You are a trained technician and you will not damage her natural nails no matter what type of enhancement you put on....but if you do take the length down on these longer nails just explain that...and the fact that they will all be an even length under the enhancement and if she keeps her rebalance appointments her nails will all grow to the longer length when eventually you can soak off and do a gorgeous overlay on them all......HTH