My mum defeacates once in 2-3 days or even less than this since young. she is 50.is't problem with her bowel or intestine? her mouth often smells

Could be Normal: What is considered normal in the number of BM's per day runs quite a range from up to 3 per day to 1 every 3 days; so, this could be normal for her; however, at age 50, she is due for a screening colonoscopy and she should make an appointment for one soon;

Bowel movements: If she has been defecating every other day or so since a young age, this may be on the "normal end of the spectrum' for her and there is no definitive concern, especially if she has no complaints. If there was a change in her bowels and she started having complaints of intestinal issues, than she should see a GI dr. Have her see a dentist to evaluate the bad breath. Best wishes.