Depends on her hair properties. If your hair is dry and needs all the moisture/ conditioning you can get, by all means use a leave in!
I've all but stopped using leave-ins in the summer. I do weekly overnite DTs with coco oil, and use CJ curl rehab as a RO a few times a week (letting it sit in the shower for 10 mins or so each time), And very good RO the remainder of days. My hair just doesn't seem to need additional conditioning beyond what I'm doing.
Anything "moisturizing" or conditioning that I leave-in, tends to diminish the hold of my gels. In the extreme high dews, wind, rain, etc of the summer, I need hold more than anything else.
2C-3A/ Coarse/ Normal to high porosity/ SW Florida
Cleanse: *CJ Daily Fix, AIA Coco Cowash, Trader Joe's TTT condish, Shea Moisture poos
Condish: CJ Smoothing, Curly Kinks Renew, Darcy's Pumpkin, *CJ Curl Rehab
Enhancers & 2nd day products: *Okra gel or FSG, Uncle Funky's CM, CJ Pattern Pusha
Gels: MGA sculpting, Giovanni LAN, Spiral Solutions FHG, LALSG, *JC Spiralicious