Popping the question with a family heirloom

Recently we were asked by a concerned sister how a man might go about obtaining the engagement ring when it is in the possession his future bride. After about an hour of research we found there is virtually no answers for this tough question on the internet. Since most men would like to keep the proposal shrouded in mystery and intrigue we want to make sure not to ruin the element of surprise, asking the bride outright for the ring is out of the question. This was our response:

Dear Concerned Big Sis,
Does your sister wear the ring currently or is it safely tucked away somewhere? If she wears it maybe you could talk to her boyfriend and find out a week or two before he plans on asking, tell her you have a coupon for a free ring cleaning and offer to take it in to have it cleaned. If it is tucked away somewhere you could tell her you are taking some of your jewelry in for a free cleaning and offer to take the ring in for her. Does the ring come with a wedding band too or just the engagement ring? If it is only the engagement ring he could propose with the the band and they could swap it out later for the engagement ring. My friend was in the same boat so her mom ask her if she wanted to have it cleaned and 3 days later her man popped the question, she had no idea=).
Hope this is helpful,
T.