。
She was over the moon about the ring he bought her.
Literal
She [topic-は] [he [subject-が] [bought-for-her-買ってくれた]] ring [because of-で] in-seventh-heaven was-becoming.
~てくれる appears inside a relative clause: 彼が買ってくれた指輪 = 'the ring he bought (for her).' The てくれる framing marks her (or someone close to the speaker) as the recipient of the kindness — 彼が買った指輪 alone would be neutral. 有頂天 (originally Buddhist, 'topmost heaven') is now a な-adjective for 'ecstatic'; pair with ~になる to mean 'become ecstatic.'