When doing a facial consultation ask your client if they would be happy to do a glucose test just prior to the facial. The totally relaxed state can reduce glucose levels and if they're erring on the 'low' side before you start you're going to have a client in a state of hypoglycemia, and when doing a facial, chances are you and the client may not notice until you've finished!
And you 'should' familiarise yourself with what to do if you find yourself in that position - although the chances are you won't, 'most' people with diabetes are pretty well controlled.
(And you'll please note, I don't use the term diabetic, your client is a person, not a condition ;-) )