I had this happen in the exact same area on the passenger side but it was my own fault about a year and a half back. This is what I did. It works for basically any type of bumper as well.
This was a How-To I had made for another forum when I had my GSR. Take it as you will and really just be creative. It has worked for me and has not come off since and i can still remove it like stock. Still holding strong to this day. I should take an after picture of it. Bumper literally ripped off the side after being pulled from being stuck in sand. The bottom plastic splash guard/air dam also came off ( could only be pulled out in reverse or risk being submerged in water so the choice was obvious ). I had then already replaced the bottom gaurd but opted to fix the bumper.
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This is what I basically did in a nut shell.
I used a Pop Rivet Gun with the appropriate length and size rivets ( you need to measure ).
And bought a small Roll of Aluminum Sheet at Home Depot or Lowes. You can also buy small sheets that cost 35 cents ( Roll is about $7).
Use a drill with the appropriate size bit to match the size rivet you choose. Drill your holes and rivet the aluminum sheet shaped as above and make sure it goes all the way through with a bit of overhang for the rivet to grip when compressed. Drill a couple holes or dremel the bigger hole for the 10mm bolt and your done. You can also try filling in the missing section with some plastic or glue or something to make the inside more solid like
Permapoxy Plastic Weld or some type of plastic glue that dries hard. I didn't and it still holding strong although I will be doing this just to give it strength inside the metal brace and just dremel the hole where the 10mm bolt goes or use a drill in a way as a dremel.
There you go. Easy fix for plastic and fiberglass bumpers. Mine is fiberglass although I fixed my wifed Mazdaspeed 3 bumper this way as well and that one is plastic.
