Is an oxygen saturation of 69% when sleeping dangerously low?

Yes. 69% saturation is quite low. Does the patient stop breathing at night (sleep apnea)? Do they have copd? Asthma or other lung disease? These questions need to be answered. The primary care physician should be consulted and a sleep study ordered.
Yes. Your oxygen level when you sleep should be above 88%. Anything lower than this can increase your risk of irregular heart beat, myocardial infarction (heart attack) and stroke. Most low oxygen levels during sleep are due to obstructive sleep apnea but if you have lung disease your oxygen level can go low at night as well. A sleep study and breathing test should be done on you.