Reddit reviews The Teachings and Practices of the Early Quanzhen Taoist Masters (Suny Series in Chinese Philosophy and Culture)
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Used Book in Good Condition
Since you are on the Daoist sub, I'll suggest your friend to research Quan Zhen Daoism one of the two majors schools of Daoism. One of its core tenants is "Three Teachings are One" refering to Buddhism, Confucianism and Daoism mainly focusing on Daoism. But there aren't too many English resources on post Warring states Daoism but here's a really good book on Quan Zhen Daoism
https://www.amazon.com/Teachings-Practices-Quanzhen-Philosophy-Paperback/dp/0791460460
Also Chan Buddhism is heavily influenced by Daoism so he should look into that as well.
There's quite a bit written on the early Quanzhen tradition. They're really the ones who got monastic Daoism off the ground and spread it, and were once responsible for all monasteries in China! So maybe you can find something of interest to write about there. The Teachings and Practices of Early Quanzhen Masters by Stephen Esikldsen and Cultivating Perfection: Msyticism and Self-transformation in Early Quanzhen Daoism are both good academic sources, with the former being slightly more accessible.