I've been furnished with some details of the site now, enough to have a search through the planning documents. It's a complicated planning history, with numerous applications, an appeal and then permission with relaxed conditions.
In short, the latest planning decision notice that I can find is dated March 2012, and conditions 17 and 18 state:
17: A survey shall be undertaken of all trees, shrubs and hedges on the land, indicating which are to be retained as part of the development. The survey shall be submitted to and approved in writing by the LPA before the development hereby permitted commences. (bold emphasis my own)
I have highlighted in bold what I think are 2 relevant issues here - the condition makes no requirement to include adjacent trees (and does not references BS5837:2012 which recommends this as good practice). It also asks that the survey is submitted and approved before the development starts, but I can see no document on the planning pages that indicates that any tree survey was submitted.
18: The trees, shrubs or hedges indicated for retention shall be protected with chestnut pale fencing not less than 1.2m in height for the duration of the development and none of the trees shall be damaged, destroyed or uprooted without written permission of the LPA. Any trees removed without such consent, or dying or being severely damaged or becoming seriously diseased before the end of that period shall be replaced with trees of such size, species and positions as may be agreed in writing by the LPA.
This is an unfortunately weak condition...protection fencing (less robust that specified in the BS) is conditioned, but it does not state how the fencing should be positioned in order to actually protect the trees!
I would agree with treequip above - it looks like the LPA have dropped the ball on this one. The tree owner should document all this as at the very least so that the LPA cannot accuse them of damaging their own TPO'd tree. I would also recommend a valuation and then try and pursue the LPA an/or developer for some costs to cover replacement planting.