I would approach the food issue on a case by case basis. I have no problem with eating halal per se. I would have no problem being served it by a butcher. It would become problematic, however, if I was in a situation whereby eating halal could be interpreted as acceptance of Mohammed above Jesus. For me it would be enough to give thanks for the food in the name of Jesus, but if that were to cause offense to the Muslim I would have to consider if not eating at all was more constructive. It would depend.