Why would it be marfu3? If you drop بصاحبي , or more precisely صاحبي, then we'd have مررتُ بعبدِ الله, and عبد would still be majrour. If we have another verb that doesn't need the preposition, for example رأيتُ عبدَ الله, then عبد would be the object the verb and hence manSub.
As for why عبدِ الله is majrur in your sentence, it's because it's a grammatical case called البَدَل. You can look it up in your grammar book(s) and by searching the forum for previous threads about بدل - البدل.