Non-grammar-heavy, but not strictly correct explanation: に indicates the direction of the verb. In the same sense as 公園に行った means “I went to the park”, お父さんに貸した means “I lent it to my dad”.
Grammar answer: に marks the indirect object of the sentence – the indirect object is an entity that’s indirectly affected by the doing of the verb, for example, as in this case, the benificiary of “giving” and “recieving” verbs. Remember, を marks the direct object, which is the entity to which the verb is done. So in this case, you’re lending the money (= direct object) to your father (= indirect object).