Hired to capture proposal, seeking advice!

Hello! I have been doing photography for over a decade and have had a photography business on the side for most of that time. Someone recently booked me to capture his proposal to his girlfriend. I’ve always wanted to capture a proposal so I am super excited, but also super nervous due to the nature of the shoot (capturing the exact moment, locating the couple, being discreet). I am planning to bring my own fiancée along to take photos of, to make it seem like I am just there taking pictures as to not ruin the surprise for his girlfriend. Does anyone have any other suggestions as to how I can pull this off successfully? Thank you!