High side ground and low side ground (SMPS)

[Yamin]

Hi guys,
I'm designing this circuit to drive a DC brushless motor (more about this perhaps in another post) and doing some research I came across this video where he shows, how he modifies an SMPS to get 300VDC from the rectified mains and 15V from the regulated output. (for that motor it needs 300VDC, 15VDC...)
He ties the high side and low side grounds together. Now as far as I know this is not such a good idea. Thoughts?
There are lot of instructional posts/videos on making sure that you don't mix up which grounds you are measuring to when troubleshooting an SMPS and referencing to the wrong ground could take out your DMM. Could it take out your DMM?
What are the possible ill effects of tying the two grounds together?
I was hoping that you could clear this up for me. I'll include a schematic shown in that video and for him the circuit worked without anything blowing up.