他们说"他的"中文好极了。 is correct. p.s. her=她的，his=他的 ----------------------- 他们说她的中文话说得好极了。 It should change to 他们说她的中文说得好极了。（ Delete the "话"。） p.s. We use "漢語"(formal)/"中文"(informal, usually refer to written chinese) , instead of "中文話"。

他们说她的中文说得好极了。 ---------------------------- v. + 得 + ad. eg. 说得好、做得对、干得漂亮 adj. + 的 + n. eg. 美丽的花儿、好看的姑娘、聪明的爱沙尼亚人 adv. + 地 + v. eg. 轻易地相信、认真地工作 ---------------------------- At present most Chinese doesn't distinguish the difference between 「的」, 「地」and「得」, so you can use one of them as you wish except in exam.

i think both are ok if you want to stress his spoken