I have applied for my mom's greencard in US in May 2018 and I applied for advance parole since she wants to travel to India (as it is going to take a couple of years atleast for the GC to come through). I was also told that she will not be able to stay for more than 3 months in India if she travels on Advanced parole. My question is if she wants to withdraw her GC processing, can she re enter US on her existing tourist visa or does that become visa become void since she applied for GC processing
Go talk with an immigration lawyer immediately. This is not something you want to do on the advice of random internet strangers.
As Musonius notes in comments, your mother has signaled immigration intent by filing for a green card. This will make it very hard to get a non-immigrant visa or VWP/ESTA as you have to show you have no immigration intent, which you can’t (easily).
Ask a lawyer. Assuming she wants to live in the future in the USA as a permanent resident, the American Immigration Lawyers Assoc has a referral database. Many lawyers will do an initial telephone interview for free and might be able to give advice as to whether this is even a possibility.