Hi. Welcome to Just Answer. My name is***** Thank you for your question. If a 90 pound dog ate 650 mg of ibuprofen, this wouldn't necessarily be a worrisome situation. Typically if doses less than 50 mg / kg are eaten, then gastrointestinal upset is the typical problem - vomiting. She is 40.9 kg so that makes this a possible 15.8 mg / kg dose. If there is a chance she ate more ibuprofen or weighs less, than this increases that dose. If she is on oral steroids or other NSAID's, then this could be worse as doses >50 mg / kg can cause stomach ulcers. So for a typical healthy dog with no underling problems and not on any other oral medications, one at 90 pounds eating 650 mg ibuprofen shouldn't be a problem.
Sorry for my delay as a case at work pulled me away. The fact that she was on the metronidazole shouldn't make things any different. AS far as signs of GI effects - vomiting is the big one. This can happen to any owner and I've actually seen it a lot at my ER! Don't feel your the first or the last to have something like this happen!