Is the parenting consistent with the father and mother, or does the father undo everything mum is trying to do? If you gently explain that the growth hormone kicks in at night perhaps both parents will get together and solve the Problem from the infants perspective.
My godchildren were all nightmares at night for their parents, tantrums, throwing toys, refusing to go to bed, screaming till they were sick, and so on ........ All slept through when staying with me. How long? Average of 3 nights if memory serves me right. Definately no longer to solve the problem. Strict bedtime routine, no TVs after tea (even if too young to watch it stimulated them too much when it's in the same room) lower the lights, up to bed, bath and teeth clean, story in bed, lights out and leave the room. Never ever bring them back downstairs.
On the odd occasion night time waking presented itself there was. A strict rule of no lights put on, no drinks given ( only plain water) no lifting up. We did find that a bit harder but again it did not take long. Not easy to do any of this when you are so tired, and it may need the infant being at home for a three week stretch until this is sorted out, mood luck!!