Surely the contract is with the original pro photographer, his price would have included the shoot as well as providing a specific number of images with any additional photographs charged for separately. I would find it hard to believe there are no images that the bride would not find acceptable.
The bride should pay the original photographer for the contracted work, providing of course they are not sub standard. There would be no obligation for the bride to buy any additional images from the pro photographer leaving the guest photographer to supply photos to the bride at whatever cost is agreed.
Remember you asked for a favour and got invited to assist the pro at the wedding, he thought he was helping you. Never look a gift horse in the mouth.
I mostly agree. However, if the guest photographer's images had been presented along with the main shooting pro's images, the bride might have chosen some of each shooters images and been satisfied with the amount of images included in the package (in the scenario described above). In that case, perhaps she wouldn't need to spend more money.
I'm not a wedding photographer. I shoot live theater and headshots. When I shoot a show for a theater, occasionally someone from the production's creative team will be shooting behind me...usually the set designer. Would it hurt me if the theater chose some of the set designer's images? Not particularly, because he's part of the team. He just needs to stay out of my way. I'm hired to make sure they get the specific images they need that are then published in the newspapers and websites that review these shows. These images are given to the press when the shows are reviewed. However, I can't imagine a scenario when I would invite someone that I don't really know to shoot along side without having some advanced agreement regarding the other shooters images.
I've been asked a few times by friends and family to shoot their weddings. They've seen my work and think I'm a great photographer. But I've never accepted. I wouldn't want to let them down. Wedding photography, to me, requires specific experience and anticipation for the event that I don't have. I couldn't expect to do a great job without getting experience first. Similarly, it wouldn't be easy shooting live theater for the first time. Shooting a musical is like shooting basketball but with constantly changing lighting.
I guess I'm saying that the OP should be grateful for the experience that the pro allowed him to receive. I don't know what was communicated between them before the wedding, but you'd think they would've had to discuss a plan just to stay out each other's way.
Another thought that came to mind...according to the OP the bride liked his images better. Does a wedding photographer show the bride all of the images prior to post processing then only process the images chosen? If so, could this be a reason the bride likes the 2nd shooter's images better? Maybe his were already processed. I can shoot 2000 images during a musical. Most theaters get jpegs right out of my camera that night. They'll make their choices for press the next morning and I'll PP the images they choose, but I'm not going to process 2000 images knowing that they will only choose 30.
If the OP is as good as he indicates, then he should be able to book some paid gigs in the not too distant future. He now has images for a portfolio that will enable him to market his work. The revenue for one job will far surpass the couple hundred bucks he might get for the 30 images this bride likes. If he takes the money and snubs the photographer that gave him the opportunity to learn, his reputation might prevent him from getting that job. Will it have been worth it?