It is a big mistake to ask her to dinner at the same time as her ex-husband! 

参考译文：在一个与邀请她前夫吃饭相同的时间里去邀请她吃饭是一个很大错误。

补完整后句子为：

(1)It is a big mistake to ask her dinner at the same time(when/in which=as) her ex-husand (is asked to the dinner)（定语从句：as在定语从句作时间状语）

(2)It is a big mistake to ask her dinner at the same time as her ex-husand (is asked to the dinner)（比较状语从句，as在比较状语从句作时间状语）

这个句子是什么意思？as 后面省略了什么东西吗？

以上解答关键在于as在句中用法，有以下2种观点进行解释。

(1)按照传统语法观点as引导定定语从句，其先行词time被限定词the same修饰限定时，用关系代词as引导定语从句。

She is wearing the same coat as I lost a few days ago. 

她正穿着跟我几天前所丢失的上衣同样的上衣。（as做宾语）

This is the same bag as I lost yesterday.

这个书包和我昨天丢失的相似。（as做宾语）

She has the same fair hair and blue eyes as her mother had.

她和母亲一样有着同样的金黄色的头发和蓝色的眼睛。（as做宾语）

(2)按照现代语法观点as引导比较状语从句，同时as要在比较状语从句充当成分.

We rode the same way as we had come out the evening before.

我们乘车行驶在前一天晚上出来的同一条路上（as做方式状语）。

A compromise was reached on the same day as the charter was granted.

达成和解和授予特许是在同一天（as做时间状语）。

Meet me at the same place as you did yesterday.

请到昨天见我的那个地方来见我（as做地点状语）。

It is a big mistake to ask her to dinner at the same time as her ex-husband!

分析：at the same time as 短语的省略原则上应与对应的比较成分在结构上一致，所以原句如果不省略应该是这样的：

It is a big mistake to ask her to dinner at the same time as (to ask) her ex-husband (to dinner)! 在邀请她的同时还邀请了她的前夫一起吃饭，这是大错特错。

类似的例句如：

I arrived at the same time as you (did). 我与你同时到达。

The horse’s mane can be washed at the same time as his body (is washed). 马的鬃毛可以随其身体一起刷洗。

The vaccine can be given at the same time as injections against diphtheria and tetanus (are given). 这种疫苗可以同时注射预防白喉和破伤风。

跟她前夫同一时间约她出去吃晚饭，真是一个大错误。

as 比较状语从句，补全是 the same time as his ex-husband ask her to dinner. 比较项是状语，在主句里是 time，从句中空缺（或认为由 as 担当）