If parents want to change things, it's actually quite a quick process. Parents need to work together and support each other, as it can feel awful for them. It's much worse for the parents through the changes than it is for the child.
At age two a child can understand what's expected and that's where I would start. Explain in simple terms what's going to happen at bedtime.
It's really important to have a strict bedtime routine at this stage and to stick to it. This could be teatime, a drink of milk, a bath, teeth, pyjamas on and into bed and then story time. By sticking to the routine the child knows what's coming and what's expected.
The parent should then say night, night, time to go to sleep now. If the child gets out of bed they should be put back in calmly without any fuss whatsoever. The parent should then leave the room.
This will have to be repeated many times but eventually a child will get tired and will go to sleep. It's essential to stay calm throughout and keep putting the child back in bed. Have a set phrase, night, night, time to go to sleep now. Don't enter into any other dialogue with the child and don't look at them. The idea is to be as boring as possible and to not reward the child in any way for their behaviour.
Some children become hysterical and may even be sick, which is extremely difficult to get through. Again, keep calm and stick to the routine.
If the child wakes in the night, repeat as above. The first night will be horrendous but over subsequent nights the resistance from the child will reduce. If both parents stick to this routine the child will start sleeping in their own bed.
Children thrive if their parents are consistent with them and set the boundaries, along with lots of love and positive attention.