I saw you had a lot of advice to move the baby out of the crib and into a toddler bed or a mattress on the floor. I, also, think this is very important, especially since the child refuses to stay in his crib. Safety is most important.
First of all, with the 18 month old, I would try putting the mattress on the floor and adding a night light or a lamp in the child's room. I would, also, make a point of spending some time in the room during the day with the child and try reading a bedtime story in the room to make the child more comfortable there. If this didn't work, then I would try this other method, which I would definitely try with the 2 1/2 year old.
I haven't tried this myself (haven't needed to) but I've had friends tell me it worked for them. Tell your friend to try putting the crib mattress/twin mattress/sleeping mat on the floor in her room by her bed. Put the child to bed there for awhile. Since it is a new house, that is probably part of the problem with the younger child. After the child transitions out of Mom and Dad's bed to the mattress/sleeping mat slowly start to move the mattress/mat and child out of the room. Move the mattress away from the bed, then out into the hall or the next room gradually, until the child is back in their room. It might take a couple of weeks but it would be well worth it. They could try this with both. I would really try it with the 2 1/2 year old.
Good luck to your friend.