Answer: This is a good question, as it can be complicated under special education law and regulations. A detailed publication about this issue is available here.
The very short answer is that dad’s friend can make special education decisions, since he essentially meets the IDEA’s definition of “parent” at this point. Your district’s Designation of Responsible Adult form probably helps to make that official for education purposes.
A liaison should not make special education decisions, as it could be seen as a conflict of interest (a school district employee making decisions about costly special education services— you could see where the appearance of a potential conflict could arise). However, liaisons can function as “temporary surrogate parents”, just for unaccompanied youth, while the district goes about designating an appropriate surrogate parent.