If the 28 is the length (side to side) not height, then that´s ok.
The tall boxes are nice, but hard to find ones that are also wide enough to handle a full clutch. Remember they can have as many as 7 chicks.
If the clutch is going to be a controlled clutch, for eg. two chicks, then having the 28cm tall box is ok.
Usually, even if they are meant to be the 28cm tall, you can take the hardware out and turn them on their side.
Chicks get big, fast. Plus mum and dad need to fit in there - though dad may spend a lot of time standing guard outside the hole when not needed to help feed hen and chicks, he will go in there at some points and huddle in with mum and the little ones.
A friend of mine wants to breed her lovebirds for the first time
Please tell you friend to create an account. It could prove helpful for her to have access to info RE breeding.