How to give advice to parents (without being a jerk), first be a friend. Make the main thing the main thing. Cultivate parenting relationships with people who raise differently. Children's environment has an effect on their behavior, so you may be able to change that behavior by changing the environment.
If you're constantly saying no to your 2-year-old, look for ways to alter your environment so that there are fewer things forbidden. This will cause less frustration for both of you. Going backwards is hard for parents, but ultimately it's what's best for our children. Talking about ourselves, answering a question with another question, using respectful phrases and avoiding saying “I told you everything” can help your child make good decisions without you.
Agree. You can find a part of the advice that you agree with. If you can, offer full agreement on that issue. If they're simply feeling overwhelmed, a little practical help, a meal, or a little child care could be more beneficial to their parenting than any advice.