Usually you just need to remove all the bar-tape and original levers and cable & housing. Remove cable from housing, then chop housing where you want to place the cross lever. Install cross-lever first, then regular lever. Install housing in between, thread through inner cable. Then final housing section to brake-caliper. Clamp cable to caliper and re-wrap handlebar.
Important thing to remember is to place the lever kinda high, almost as high as the bars. That way, when she uses them, her palms are BEHIND the bars, not ABOVE. Having your hands above the bars means you only have your thumbs behind the bars to support your weight flying forwards. You want to use them like MTB levers, with your palms behind the bars.
I've had plenty of customers come into the shop to have those levers removed because they broke their thumbs in a panic stop and they STILL
ended up flying over their bars and crashing. One of them actually ended up under a car; luckily it stopped in time before running them completely over. Talk about adding insult to injury!