I would suggest to keep it simple.
In terms of moisturizing shampoo:
Creme of Nature shampoo (blue label for fine hair), Motions conditioning shampoo (there are ones formulated for fine hair), Mane 'n Tail Shampoo
or any shampoo that says conditioning or moisturizing shampoo.
In terms of conditioning, she can try:
ORS Olive Oil Replenishing Pak, Dark 'n Lovely Dry Hair Healer conditioner, LeKair Shea Butter Mango Butter conditoner, Elasta QP DPR-11, Mane n' Tail conditioner. Motions Silk Protein conditioner or CPR, etc. Some of them contain protein which may make the hair even drier. So she should use this like every three weeks.
Hair moisturizer: BB Castor Oil and Aloe Vera moisturizer, Hollywood Beauty Carrot Oil or Olive Oil cream, ORS Carrot Oil Cream or Olive Oil Cream, Sta-Sof-Fro, 360 Afro Hair softener spray, Black n' Sassy Carribean Leave-In conditioner, coconut oil, olive oil (one by Palmer's)
This is a just a suggestion. I hope some of them will help. Just remember your cousin may not see improvement but if she keeps using some of the suggested products, over time her hair will improve. She also might need to change some hair styling techniques. Good luck to your cousin.