What is considered too low for diastolic pressure?

Depends: "too low" implies that it is a problem. 70 is lower than average but would not raise an eyebrow. 50-60 is unusual and would lead me to think, why? If you feel well it is probably not a problem. But if the diastolic pressure is on the low side, i would consider things that might cause that even if the patient is well. Leaking of the aortic valve for instance can cause a low diastolic pressure.

Diastolic BP: Normal range for diastolic blood pressure is 60 to 80 mmhg (mm of mercury). Diastolic BP less than 60 is low though most people will only start to feel symptoms at 50 or less.