If you hit or scraped something there will be evidence somewhere in the plastic, it's soft enough it almost always marks. That soft plastic pretty much does not accept any glues well, the parts come loose again really quick. They often plastic weld (melt) back supporting brackets at factory and they need to be re-attached same way. It could be there was a faulty weld there and parts simply came apart. If no physical apparent damage other than looseness I certainly would be not paying $900 for it and most especially if it just came apart by itself........I've imitated guidepins before with a bolt that I spent 5 minutes on bench grinder to make custom one off part, just depends on what is needed there. Lots of times they use a backing plate on backside of cover, that plate slides onto guidepin, if plate comes loose from cover then the bumper cover flops loose at the side. Since plate is considered part of bumper cover they will simply buy and repaint another new cover, very wasteful for what could be an easy cheap fix, unfortunately they are not interested in that. Their aim is to generate as much money as they can from a job.
$900, that's pretty funny, but they rely on you not having the gumption to repair it for maybe $20-$30 yourself............it's the dealer way. They have to pay for that glass showroom somehow.
I fixed my son's '12 Nissan bumper and support when some jerk sideswiped it in restaurant parking lot. Corner of bumper pushed in and turned inside out, luckily during summer when good and hot. I used a heat gun to warm it even more and simply re-pushed back out the other way bit by bit with bumper cover removed from car. It tore the quick snap fasteners in 2-3 places, I drilled holes in unseen places and riveted back together with pop rivets. Touch up paint at $7/can and entire repair took maybe 3 hours and $10, the estimate was $1500. Workers where he worked next day could not tell where it had been hit. That's close to a $500/hr, pay rate............