Ok , I’m now convinced. The Junior Boy won that hole. Area Handicap Advisor’s response.
For any mixed tee event you use an 'own ball/own par/SI' situation. For example, 4BBB or singles m/p, the player uses their own SI for giving/receiving strokes. If you share a ball (foursomes), you use the forward par/SI.
In this situation, as you describe, there are no strokes to be given. So whoever gets the best score for the hole will win.
As strokes have been allocated at the beginning of the round to take the difficulty of each course into account, there is no other adjustment required.
Consider: if you were playing another man, both off yellow tees, it is simply the best net score which will win. If you had a 5/4 and Fred had a 4/4, it is a simple half. Nothing to do with Fred having a par, and you having a bogey.