Definitely a project fraught with lots of potntial for wrecks ! Doubt if there is enough meat to allow for 1/4 inch all thread . I would try deck screws which are about 3/16 . Might need to reduce that a bit by grinding them down .
Step 1 would be drilling the holes alongside the rivets and right along the blade , about an inch deep . Fill those with epoxy (one which takes several hours to set and a week to fully cure) , screw in the screws , wipe away excess epoxy , and let sit vertically for a day .
Step 2 Hacksaw off the screw heads , drill matching holes in the holes in the handle , about an inch deep . That will not be easy . Some chainsaw file or very careful Dremel work will be needed in the holes to allow a good fit , with likely many trial assemblies . Fill holes to about 3/4 depth ?? with epoxy , apply light coat to the broken surface , particularly around the rivets , squeeze gently together (not clamped) , and hopefully get a bit of epoxy coming out along most of the break . Could add a hint of black to the epoxy if concerned about glue line show . Cut off squeezed out glue with a razor blade after it has set , before it gets rock hard .
Make sure your customer understands the complexity of the process , and that there is no guarantee . (And tell her not to make very thick icing , or spread fondant with it . )
If the above makes it sound like I`ve done this before , please rest assured that I haven`t . It would just be my engineering approach to the problem . Good luck .