Aquarium tubing while okay for water and some liquids, is way too small to clear a crop. You can tube fluids and massage the crop and hopefully it will help liquify the crop contents. But to lavage crop contents (instill water and then pull the contents back out,) you would need a much bigger tube. I have read that vets will sell a feeding tube and a large catheter tipped syringe. You can also buy them online. I was hoping that
@casportpony would chime in and give you advice. I have never anything other than tube feed a chicken. She would have more knowledge of flushing or lavaging a crop.
Since this isn’t not your chicken, it might be better if your neighbor handles this, especially to separate and treat the chicken. Crop problems are very common in chickens and usually are a result of another problem. Reproductive disorders, worms, ascites (water belly) and other problems can cause crop disorders. I have treated a half dozen older hens, and have never had any success with sour or impacted crop. So I leave advice up to others who have had better success.