My son's ex is planning to get remarried. For the past few months, she has been letting him have time with his 3 yr old daughter less and less. She moved in with her boyfriend about 2 months ago and she won't tell my son where they are living or who she is going to marry. She works 4 nights a week at a bar in the next town and her relatives keep my granddaughter half the week everyweek. Would her leaving her daughter for several days every week constitute abandonment and would her working all night at a bar have any bearing on my son being able to get full custoday? With the lifestyle that this child leads, she will never be able to function in school when she is older.

As long as she sees that her daughter is well taken care of and she is not taking her to the bar I don't think it would have anything to do with a ruling toward your favor.
The best your son can do is go to court for a custody hearing. The Judge will most likely rule for joint custody.

It may help if he wants to try and file for phsycial custody, but it may not. Since she is working, her job is not illegal or imorral. So you will prove in court she is working to support her family ( is his child support up to date ?)

Yes, child support is up to date and thanks very much for any input.