。
She lent a hand to help the old person cross.
Literal
She [topic-は] old-person [subject-が] crossing [for-のに] hand [object-を] lent gave-as-favor.
のに here is the purpose sense ('for the purpose of crossing'), not the contrastive 'despite.' 手を貸す (てをかす, 'to lend a hand') is an idiom meaning 'to help out.' The ~てあげた benefactive marks it as a favor done for the elderly person's benefit. 横断する (おうだんする) is a formal word for 'to cross (a road).'