Diabetes and Low blood sugar

My Sis-in-law is having diabetes for many years and she continously monitors sugar levels and takes injections twice a day. But yesterday her sugar level fell drastically and she almost fainted. So to raise the sugar level she had a glassful of milk and some sugar. The result was that by the evening sugar level shot up to 300.
These drastic changes are worrying . Is it possible that the drop in sugar levels was due to something other than diabetes?
How to maintain stability? Should she get her thyroid tested?

[WaveRider]

"But yesterday her sugar level fell drastically and she almost fainted."
Were you present when this happened? What time of day did this occur? Did she take insulin and not eat soon after? How did she know her glucose "fell drastically"? Has this happened more than once?

"So to raise the sugar level she had a glassful of milk and some sugar. The result was that by the evening sugar level shot up to 300."
When did she consume the milk and sugar and how much? Is this "all" she consumed to raise her glucose levels? Did she continue to consume sugary products into the evening?

"Should she get her thyroid tested?"
Yes and also her pancreas insulin production. Also a full health examination and evaluation by her doctor.

"How to maintain stability?"
With lifestyle changes; proper diabetic nutrition, maintain proper body weight, do 30 minutes of physical exercise daily, maintain proper levels of cholesterol, triglycerides, and blood pressure.