"νἀ βοηθηθἠ" is written according to an older spelling rule. Maybe the book in which you found the phrase follows an older (not the current) spelling system, and maybe you have noticed also other differences and variations that seem unfamiliar to you. (for example accent mark over one-syllable words.)
Nowadays it should be written as "να βοηθηθεί" (ει and not η in the ending of the verb).
According to the older spelling system, verbs in subjunctive ( that means with να before them) take η and not ει in the ending of the second and third singular person in active and passive voice.
Older spelling: να βοηθήσης, να βοηθήση (second and third person, active)
να βοηθηθής, να βοηθηθή (second and third person, passive)
Current spelling: να βοηθήσεις, να βοηθήσει
να βοηθηθείς, να βοηθηθεί.