Is broken hip serious at 70?

Yes. Yes I would say it is serious in the sense that many hip fractures do require surgery and there is certainly some recovery involved as well. That being said, many patients do have hip fracture, undergo surgery, and then ultimately do quite well.
Yes. A broken hip at any age is serious. It is a reflection of the overall condition of a patient. It often means that there is enough going on to allow a patient to be off balance, to be lightheaded, to have weakness, and to have osteoporosis. If none of these apply, then the hip is just broken because of dumb luck and we fix it and move on!