The question of:

> What are the possible repercussions of developing an application for a customer that will violate the TOS of another company?

is one that is purely a legal one. The possible repercussions are whatever the lawyers at the company dream up.

As this is a legal question, it is not one that we, as programmers, can answer and is thus off topic under the category of 'legal assistance' as described in the [help center](http://programmers.stackexchange.com/help/on-topic).