If your parent can get a letter from the Party Group Leader addressing that your parent has not paid the Party membership fee and not participated in the Party acitivities for many years (had better specific number, e.g. 6 years). That means your parent has quitted the Party based on the Party Constitution (党章）。That will be very helpful.
Because the hostile atmosphere between China and US in these days, your parent's Party membership may have negative effect on the I-485 approval. Of course, it is case by case.
