Simplify the issue
They probably wanted to save money so they didn't buy the highest resolution version of the image. I bet their is a higher res version available for purchase on whatever stock photo website they bought it.
100dpi (even 72 dpi) for a wall mural "should" work.
Option 1.) Client purchases higher res version of that file
Option 2.) Client buys a similar image with higher res.
Option 3.) That's it, don't screw around with trying to backdoor a solution or manually re-touching the photo. In the end, you'll spend a lot more money and the client will be upset with how much time it takes, or extra cost, etc.
Don't over complicate it. Get a higher res image and print away.
They probably wanted to save money so they didn't buy the highest resolution version of the image. I bet their is a higher res version available for purchase on whatever stock photo website they bought it.
100dpi (even 72 dpi) for a wall mural "should" work.
Option 1.) Client purchases higher res version of that file
Option 2.) Client buys a similar image with higher res.
Option 3.) That's it, don't screw around with trying to backdoor a solution or manually re-touching the photo. In the end, you'll spend a lot more money and the client will be upset with how much time it takes, or extra cost, etc.
Don't over complicate it. Get a higher res image and print away.