Extension methods for modify the PhysicsMaterial2D on Rigidbody2D/Collider2D components without it affecting other objects.

Physics2DExtensions.cs

using UnityEngine;

namespace Invertex.Unity2D.Physics
{
    public static class Physics2DExtensions
    {
        //Example Usage: rigidbody.ModifyUniquePhysicsMaterial2D(0.5f, 0.2f);

        /// <summary>
        /// Sets the given values on the PhysicsMaterial2D of the Rigidbody. If a material unique to this object doesn't exist, a new one will be created just for it.
        /// </summary>
        /// <param name="rb">The target Rigidbody to change the Physics Material 2D of.</param>
        /// <param name="bounciness">Modify Bounciness. Default value less than 0 will result in existing value being kept.</param>
        /// <param name="friction">Modify Friction.  Default value less than 0 will result in existing value being kept.</param>
        public static void ModifyUniquePhysicsMaterial2D(this Rigidbody2D rb, float bounciness = -1, float friction = -1) => rb.sharedMaterial = ModifyUniquePhysicsMaterial2D(rb.sharedMaterial, rb, bounciness, friction);

        /// <summary>
        /// Sets the given values on the PhysicsMaterial2D of the Rigidbody. If a material unique to this object doesn't exist, a new one will be created just for it.
        /// </summary>
        /// <param name="col">The target Collider2D to change the Physics Material 2D of.</param>
        /// <param name="bounciness">Modify Bounciness. Default value less than 0 will result in existing value being kept.</param>
        /// <param name="friction">Modify Friction.  Default value less than 0 will result in existing value being kept.</param>
        public static void ModifyUniquePhysicsMaterial2D(this Collider2D col, float bounciness = -1, float friction = -1) => col.sharedMaterial = ModifyUniquePhysicsMaterial2D(col.sharedMaterial, col, bounciness, friction);
        
        private static PhysicsMaterial2D ModifyUniquePhysicsMaterial2D(PhysicsMaterial2D curMat, Object target, float bounciness = -1, float friction = -1)
        {
            //Only replace the values if they're valid/non-default
            bounciness = bounciness < 0 ? curMat.bounciness : bounciness;
            friction = friction < 0 ? curMat.friction : friction;

            //Instance a Unique copy if one isn't already assigned, so we don't modify all objects sharing the material
            var instID = target.GetInstanceID().ToString();
            if (curMat.name != instID)
            { //Avoid instantiating a new material everytime we modify the same object again
                curMat = new PhysicsMaterial2D(instID); //
            }

            curMat.bounciness = bounciness;
            curMat.friction = friction;

            return curMat;
        }
    }
}