"Cross Country" can have different definitions depending on what the cross country is being applied to.
In it's most simple definition, it's any flight that has a landing at an point other than the point of deaprture.
You are correct that for the purposes of a person working on their IA rating, that the >50NM rule applies. Show the CFII that is questioning this that 61.1(b)(3)(ii) contains the wording "for the meeting the requirements for an instrument rating"...and then (B) of that same section specifies the >50NM requirement.
The other CFI that is saying 25 NM is getting confused with the language in 61.93 that is used for STUDENT PILOTS. Although the person working on his IA is an "instrument student", he is not a "Student Pilot" in the FAR sense.
If the CFII's still want to argue the point, get a copy of the FAR, highlight the applicable portions mentioned above and ask them to defend their positions to you.
