You might like to read also another bit of Peter Higginbotham's splendid work http://www.workhouses.org.uk/life/entry.shtml which explains that people weren't actually put in the workhouse but entered of their own volition because of whatever unfortunate circumstances they found themselves in. A possible scenario might have been that she left him as your grandfather tells the story, or he left her (with another family perhaps that is likely) and whatever, she had no means of supporting herself and had to apply to enter the workhouse.
maxD
The story is that he paid a docker to say he had been having an affair with her so he could blacken her character, not sure if she knew of his wanderings at that time. The women had an illegitimate child and was married off to a widower after the birth. Her birth certificate said father unknown but we think he was the father. She then had two more children whose names were registered as Lewis.
My grandfather only saw his mother once after she left that's how we know that she was in Salford and it was not long before she died. There shouldn't have been any reason for her to be penniless, he had owned a fishing vessel, the home they lived in and the house that his other family lived in. He wasn't able to marry again until Mary died in 1925.
Many thanks.