cuidam hosti is dative because of the verb ademerat. This use of the dative, if you're concerned with labeling things, might be called the Dative of Separation. The Dative of Separation will often go along with a verb that indicates something along the lines of "to remove from (somebody), to take away from (somebody)." The (somebody) in these cases are put into the dative. Also of note, these verbs are frequently compounds with ab, de, ex, and a few of ad, such as ademerat.
See A&G 381 ("Dative of Separation") for more details.