tone sandhi represented in Pinyin According to Wiedenhof's A Grammar of Mandarin, page 25,


  When following a frst or a second tone, and simultaneously preceding a
  non-neutral tone, a second tone is pronounced with a high level pitch
  55 (high to high), i.e. as a frst tone. This diference can be represented in  Pinyin: ānmián yào ~ ānmiān yào ‘sleeping pills’.


Does "represent" here mean that, instead of the citation/lexical tones, the new tones after tone sandhi can be used as well?
No, this is not standard Pinyin, if this form of tone sandhi is written out. The author is simply trying to convey what it would look like if written out (instead of retranscribing it with Chao-style tone letters).

This type of sandhi ("Mandarin T2 Sandhi", for Tone 2 Sandhi), is mentioned only rarely, because it is a "prosodic-driven process", i.e. it happens because the syllable is not in a strong position in the overall 'rhythm and melody' ( = prosody) of the utterance.

However, T2 sandhi is not an ingrained feature of standard Mandarin or even any one dialect of Mandarin (unlike T3 sandhi), but a "variation", and one that is far from universal.