The Good Friday Agreement requires "rigorous impartiality" by the UK government, so it can arbitrate between parties in Northern Ireland. A UK government which depends on one of those parties for its existence may be breaking that rule.
Whether the UK government is breaking the agreement can only be officially settled by a court of law. In principle it could be resolved by the UK Supreme Court, or even an international body such as the International Court of Justice (since the agreement has the status of an international treaty).
At present (2017-06-16) there is no formal agreement between the Conservatives and DUP, and a new UK government has not yet been confirmed in office by a Parliamentary vote, so it is too early to tell whether there are legal grounds to challenge the Conservatives' actions.