How does one avert jealousy from a mother-in-law whose issue is that her son spends more time with his wife than with her?
I don't know that you can "avert" jealousy. Sounds like it's more her issue than yours....she needs to find something else to do with her time.
You have your husband explain to her that she was successful in raising her son. Her job was raise him to leave the nest and nuture a family of his own and now he is doing just that. Then he needs to put her on a schedule. If she lives nearby, he can do dinner on Tuesday nights or something. If she doesn't live nearbby you set up a phone call schedule. But it will be up to your husband to set the ground rules, and then stick to them.
First 3 answers are all good.
I must admit I would just like to tell her to "Grow Up."
I wonder how her husband treated his mother relative to her when they were first married.