La fille d'Henri, de laquelle je t'ai parlé,

Benjamin, 

there is agreement in gender and number of the relative pronoun with the preposition-noun it replaces - it is  ‘ de laquelle ‘ because it refers to one female.

However, that is not the same as invoking the rule of agreement of past participles of compound verbs with auxiliary avoir ( avoir parlé ) , which requires the verb’s direct object precede it in the phrase. 

There is no direct object here as the verb form is ’ parler de qqn à  qqn ‘. 

In ‘ Je t’ai parlé…. ‘ , there is a contraction of the indirect object pronoun ‘te’ with ‘ai’, and past participle agreement does not occur with this part of the structure either. 

 Special cases when the past participle agrees (in number & gender) when used with 'avoir' in the compound past in French (Le Passé Composé)

 https://www.lawlessfrench.com/grammar/verbs-with-a/