Lynn could not see Bruce Willis. She did not know this ghost was helping her son through his crippling emotional problems. She watched her kid this whole time screaming at birthday parties, writing weird shit, acting constantly tense and getting hospitalized and she didn't do a single goddamned thing. During Cole's stay at the hospital, she is actually told by a doctor there that the boy needs help. And still she does shit. Seriously, watch the film and mentally remove Bruce Willis, and you see that this kid is left to twist in the wind.
All those times when we see Cole walking down the street, taking the bus to the funeral and being greeted after school by Malcolm, he is actually doing those things with nobody. Nobody actually saw him perform in the school play, nobody came to see him after he got called a freak by his teacher at school, nobody escorted him on the bus to some random neighborhood to some random funeral.
To the rest of the world -- and his mom -- he was just some kid walking around and talking to himself. Lynn needs to thank her lucky stars that Cole's ghost friend happened to be real and happened to be a child psychiatrist, because otherwise, well, we know the alternative ...
Cole, 15 years later.