。
She was praying with all her heart.
Literal
She [topic-は] single-mindedly was-praying.
一心に ('single-mindedly, with one heart') is a fixed adverb built on 一心 ('one heart/mind') + adverbial に. Common with verbs of focused action: 一心に祈る (pray), 一心に勉強する (study), 一心に願う (wish). Past progressive 祈っていた frames the praying as an extended scene the speaker observed — not a one-off act. 祈る in Japanese covers both formal religious prayer (at shrines, temples, churches) and everyday wishful 'I'm praying that...' sentiments.