你问：once是表示“有一次”还是“曾经”，其实 once 是表示“有一次”，其实也是表示“曾经有一次”，你说的“曾经”可能是指“曾经一度”。比如：

I once went to town to see him, and he was out. 有一次我进城去看他，但他外出了。（曾经有一次）

She was once my best friend. 她曾是我最好的朋友。（=曾经一度）

一般说来，如果修饰终止性动词，多半表示“曾经有一次”。如：

I once went there for a holiday我有一次去那儿度假。

She once heard Xiao Li talk about it. 她有一次听到小李谈到过这件事。

He had once fought with these rascals. 他有一次和这些坏蛋打过架。

I once took part with him in a debate. 我有一次在讨论中站在他一边。

I once brought a fellow to that was drowned. 我有一次曾经把一个溺水的人救活。

She was once attacked by a crazed fan. 她有一次曾受到一个狂热崇拜者的袭击。

For a bet he once rode 200 miles in nine hours. 为了打赌，他有一次用九个小时骑了200英里。

Once we drove all the way to Edinburgh with four little kids in the back seat. 有一次，我们让4个孩子坐在后座，一路开车去爱丁堡。

如果修饰持续性动词，多半表示“曾经一度”。如：

He once lived in Geneva. 他曾经一度住在日内瓦。

France was once a colonial power. 法国一度是占有殖民地的强国。

Vegetarians were once regarded as cranks. 素食者一度被认为是怪人。

It was once thought that the sun travelled around the earth. 人们一度认为太阳绕着地球转。

Diamonds were once thought to have magical powers. 钻石曾一度被认为具有魔力。

Questions and problems that were once taboo are now discussed openly. 一度视为犯忌的许多问题现在可以公开谈论了。

Greek and Latin were once viewed as the foundation stones of a good education. 希腊语和拉丁语一度被认为是良好教育的奠基石。

回到网友的句子：

Most page turners are piano students or up-and-coming concert pianists, although Ms Raspopova has once asked her husband to help her out on stage.

其实这里的 once 理解成“曾经有一次”或“曾经有一度”都不算错：大多数翻页者是学钢琴的学生或有前途的音乐会钢琴家，尽管拉斯波波娃女士也曾（有一次，一度）请她的丈夫在舞台上给她帮忙。