J.H.
Wow, here you have twins who have been raised together from the womb, and one is a good sleeper and one is not. This just proves my theory that some kids are good sleepers and some are not.
I myself have a not-good sleeper. She's also 2.5. The other responses seem too harsh to me. Parenting is unfortunately a 24/7 job. I believe that kids need to be parented at night, too.
That said, here is what we do with my daughter. She gets a bath, teeth brushed, story or two read, and then we turn out the lights and rock her until she goes to sleep. When she wakes at night, we either lay back down with her or allow her into our bed. This feels right to me. We all get more sleep and there is no struggling.
The problem with methods is that kids are not robots. They are emotional beings with unique minds and needs. I would encourage your friend to first make sure that there isn't some physical reason for her daughter's waking: molars, stomach ache, food or airborne allergies, etc. Then she should try to address any emotional issues. Is she missing her father? Has she seen something on t.v. or in real life that scared her? If she's at daycare all day, maybe she needs to be close to mom at night.
Just some suggestions. I know it's frustrating, but I believe that if we meet our kids' nighttime needs now, they will grow up to be better adjusted and better sleepers overall.