The first rule is that a conjugated verb in a subordinate clause ("weil") must be at the very end (just like in Japanese). Therefore there's no need to split the prefix off, the prefix can stay where it is. Splitting only happens when the verb is in second position in a main clause.
Main clause: Seine Freundin nimmt ihn zur Uni mit.
Subclause: ..., weil seine Freundin ihn zur Uni mitnimmt.
"..., weil seine Freundin nimmt ..." is wrong, because the verb is in the wrong position. Any other verb in this position would also be wrong.