。
Friends must help each other.
Literal
Friends [topic-は] help-each-other [must-なければいけない].
助け合う uses the reciprocal suffix ~合う — 'help mutually.' ~なければいけない is a variant of ~なければならない, both meaning 'must.' いけない (literally 'it will not do') is slightly less formal than ならない but carries the same obligatory force.