i've fixed crash damaged bodykits before your best bet is to bond in some thin stainless struts to hold the 2 parts together, depending on the gap size (more than about 10mm) i would also use some tiny aluminium mesh (can be found in halfrauds in the repair section.) basically put the mesh over the struts to give you a solid surface to bond filler on to.
then fiber glass over the whole lot on the inside. if the gap is only tiny use something like filler , if its bigger use something like isopon p45. sand and fill with a fine body filler, more sanding and filling etc etc.
thats the diy job,
a proper bodyshop would trim back the layers of fibre glass in a step pattern then fill and cross lay between them and finish with gel coating top. dpending o the break, they might brace as well. and also layer up behind.