What is the legal limitation for exceeding the value of a contract for scope determination and what reference can this be found in?
The GAO rulings your friend mentioned were where GAO approved 25-30 increases as within scope- but that should not be construed as a ceiling.. The GAO has consistently ruled that in scope or out of scope determinations are judgment calls to be made by the contracting officer.
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Here's one GAO decision that covers the issue:
GAO has viewed any analysis of this issue as one to be determined by examining whether the alteration is within the scope of the competition which was originally conducted. Ordinarily, a modification falls within the scope of the procurement provided that it is of a nature which potential offerors would have reasonably anticipated under the changes clause. To determine what potential offerors would have reasonably expected, consideration should be given, in our view, to the procurement format used, the history of the present and related past procurements, and the nature of the supplies or services sought. A variety of factors may be pertinent, including: whether the requirement was appropriate initially for an advertised or negotiated procurement; whether a standard off-the-shelf or similar item is sought; or to whether, e.g., the contract is one for research and development, suggesting that broad changes might be expected because the Government’s requirements are at best indefinite.
American Air Filter Co., DLA Request for Reconsideration, B-188408, June 19,
1978, 78-1 CPD ¶ 443, at 9-10.
In more recent years, however, GAO has appeared to defer to an agency's determination that a modification is within the scope of a particular contract.
Part of the reason for the shift is a change in the way many contracts are written. In the past, most contracts were narrowly focused on specific government requirements. More recently, many contracts have been awarded to address broader program needs. A modification is more likely to be found to be within the scope of a broadly written contract.
Furthermore, GAO seems to accept more readily allegedly out-of-scope modifications since the Court of Appeals for the Federal Circuit issued its decision in AT&T Communications Inc. v. Wiltel. The court found that a$100 million modification adding a new type of transmission services to one of the FTS 2000 contracts was within the scope of the contract, reversing a decision by the General Services Board of Contract Appeals.
In general, a modification is considered to be outside the scope of an existing contract when there is a "material difference" between the contract as modified and the contract as it existed before the modification. In Access Research Corp., GAO noted that: "Evidence of a material difference between the modification and the original contract is found by examining any changes in the type of work, performance period and costs between the contracts as awarded and as modified."