Originally Posted by
FBinNY
The fender brace didn't have to be strong, it was the meat in a sandwich. It was trapped when the tire was driven into the tube. If this bike didn't have fenders, he would have ended up with a broader shallower dent, or maybe no dent at all. However there wasn't enough force to bend the entire tube upward or change the geometry of the frame (as appears in the photo). The OP can confirm that with a straightedge, or by checking the front wheel position with a new fork.
Obviously none of us were there, so anything we say is opinion. It's also opinion as far as predicting the outcome if the OP chooses to ride the bike. My opinion is based on what I see in the photo, and 45+ years working on bikes.
Look more closely at the last picture. The bend in the tube is v-shaped and very deep as well as spanning nearly the entire width of the tube. No fender brace I've ever seen is v-shaped like that dent.
Additionally, a tube gets its strength from the round nature of the tube. Make a part of it 'not round' and the strength decreases dramatically. This frame now has a 'not round' tube at a critical stress point. It may not fail for days, months or years. But it also may fail at any point. Failure of the frame at that point is going help the rider realize the value of a good dentist...if not a good plastic surgeon or neurosurgeon. Don't ride it Spld cyclist. It's busted.