My wife has severe lower back pain. The pain came on gradually. She can walk with the aid of a walker. She has noradicular pain. The doctor in the ER thinks that it is a herniated disk but I think it is a spasm. Your opinion?

Herniated disc? Sounds like your wife needs an mri. Herniated disc is likely, although it could be several different things. If your wife has new weakness, bowel, or bladder changes these are never normal and need to be worked up emergently.
Back pain. Has she had studies of her back? That may be needed to clearly know what the underlying problem is. Er's are more for dealing with issues acutely vice doing the longer workups that occur over time. What does her regular doctor think? She can have spasm with a herniated disk. Where is your practice?
Needs more workup. Back pain severe enough that a 38 year old women needs to use a walker is uncommon. I would suggest an evaluation with her primary care physician. Starting with x-rays would be reasonable and if it doesn't respond to conservative management or if there are neurological issues identified on examination, an MRI would be my suggestion.