I made the mistake of asking my advisor to write a reference letter for a non-academic internship. Not only did he sabotaged me in the letter, he also started to trash talk me to other faculty members. My life is pretty miserable now. So, I don’t know about the advice of being up-front. If your advisor is a decent human being maybe, but that’s a big if.
Pay atttention retard. The advice was to be upfront ASAP, well before it’s time to ask for the letter. The entire point is to avoid your situation by finding out well in advance whether the current advisor will be supportive. It gives OP the time to find a new supervisor if necessary.