I reread this and understand now.
There's no need for him to stand as far back as the male player serving in a typical mixed doubles scenario.
Let me explain a little more clearly. A lot of people who do a doubles serve stand right up to the service line when they serve. Not sure if you partner does the same. However, not everyone can pull off a consistently good low serve from this position. Why? The closer you are to the net when serving, the steeper the shuttle has to travel up to the top of the net to go over. Thus, the shuttle has more chance to go further upwards after it has past the net. This puts a lot of pressure on the server to make a good serve. Not an easy situation even though we see the pros do it.
I found for myself, serving from about 6-8 inches behind the service line would increase my quality of serve a lot. The shuttle can travel a little flatter, the highest point of the shuttle trajectory would be the tape of the net, the shuttle would be descending straight after crossing the net.
Because of the shuttle descending, the receiver has to hit the shuttle upwards. It shifts the game dynamics so that you are more likely to have the initiative very early in the rally.
I still need to practice the serve for the consistency but just stepping back slightly from the service line when serving in doubles really made a big difference.
With the below waist rule I think it makes sense to stand back if you're a smaller player, but on the line if you're taller. Not sure if it does with the new rule. People should do what works for them though and not just stand on the line because everyone else is.