Ceasing up

Hi

can anyone think why this happens to a friend of mine.

She's slim (5'4" just over 8 stone) very fit (aerobic instructor) and very good runner 3.20 marathons.  After a run she gets cold very quickly and has to put layers of clothes on and then shes fine,BUT twice now we've done the moonwalk( breast cancer charity event) where we walk at midnight a full marathon (7 1/2 hours) wearing decorated bra.  she's fine throughout the walk then about 5-10 mins after we finish she just totally ceases up to the point she can hardly stand never mind walk, she also feels freezing, shivering and goes dizzy and her lips tinge blue, this last about 5-10 mins, the pain and stiffness in hips and lower back last about 2-3 hours, after that it eases off and usually okay the next day. Any ideas?