'to befriend' is to become someone's friend and treat them accordingly. It is more often used in the past and future tenses, than it is used in present tenses. There is no need for any preposition when using the verb 'befriend' ,the direct object stands directly after the verb without any prepositional link.

I think such mistakes comes from mothe language (the using of preposition) .this is the same in arabic language ; we use a preposition while in english we need no preposition .Mary befriended me .or mary will befriend me \Mary has befiended me .If she has already befriended you (present participle tence) this is correct .However,you can use the simple past tense to express the same meaning because simple past and present perfect here have the same meaning .SO ,Mary has already befriended me =Mary befriended me . Best wishes , Hamid

Yes, the meaning is that she is now your friend. You can say, "Mary befriended me" or "Mary has befriended me." You don't use a preposition: don't confuse this with, "Mary is a friend of mine." "To befriend" is a verb, but "friend" is a noun.