Can an LPN challenge the NCLEX-RN....

There is no state in the US that will allow people to "challenge" the NCLEX-RN. You must complete an approved (registered) nursing education program to be eligible.

If an individual is an LVN/LPN with the desire to earn the RN licensure, (s)he must complete an RN bridge program and pass the NCLEX-RN. There is no way of getting around this. There is currently no way for an experienced LPN/LVN to become an RN by bypassing school and challenging the boards.

In other words, "no pain, no gain." You must attend a state-approved school of nursing.

Your friend may have gotten confused information pertaining to the distance programs, in particular, Excelsior College. Some people might get this conception before they really find out what the deal is with distance learning for nursing. I can assure you, if your friend's statements were correct, there would be a dither more RNs running around than there are currently. That's for sure.

While I do suspect that many LPNs are able to actually pass NCLEX-RN because of their critical thinking skills, experience, and excellent test taking skills, there is no way that they would allow (that I know of) an LPN that has not had formal education to sit for the exam. An accreditated school must send records to the state board of nursing stating that the student has successfully completed their RN program for a candidate to even be allowed to apply for the test.