1. 该句存在省略,完整句子为：
She is looking forward as much to his return as he himself (is looking forward) to seeing her.
2. 即句中as引导的比较状语从句省略了谓语is looking forward (to),而该短语中to为介词,后面接名词/动名词,所以用seeing.
它刚好也和前面的名词短语his return一样,都是(look forward) to的宾语.