My fiance is a quad. As a result of an auto accident. Are there any suggestions on how to help prevent kidney and bladder infections?

Clean cathing. The literature is rather clear on the fact that nitrofurantoin, methenamine, vitamin c or cranberry pills do not lower the risk of utis anymore than placebo. The risk of chronic Nitrofurantoin use is pulmonary fibrosis which is to be avoided at all cost, especially in a tetraplegic person. Use clean cathing to avoid contamination and cath often to prevent urine stagnation and bacterial growth.
Bladder emptying. The same nerves that control the muscles in the legs, also control the bladder and bowel. Your fiance needs a full work-up of his bladder functioning to determine if he is empying his bladder and if the pressures are high and damaging his kidneys. He should void every 2-3 hours, increase fluid intake, and see a urologist who specializes in neurogenic bladders.
UTI prevention. Nitrofutantoin taken 100 mg daily can prevent recurrent uti's.
Intermittent caths. Clean intermittent catheterization 4 - 5 times/day and prophllactic antibacterial like Macrodantin (nitrofurantoin) after course of antibiotics if he gets a uti. Cranberry gelcaps or tablets may reduce risk of utis. "probiotic" yoghurt, with live culture may also help. Do not let bladder get overdistended. Maintain satisfactory bowel regime to avoid constipation. Good luck to both of you.