thoongeng's answer is more scientific...yes by calculation.There is a difference between scaling a photo to maintain aspect ratio and cropping which may not be the same aspect ratio of original image.
I guess TS has cut some part of the image.The 12Mp is what the size of the image file and may not be the actual resolution.What is more relevant is the height and width of the cropped image given
by the pixel or dot numbers (check image size info) so a more accurate enlargement as shown by thoongeng.
Definitions : DPI refer to a printer's resolution of printed dots on 1 inch of line.
PPI have 2 meanings...1. refers to resolution of LCD monitor that can not be changed.
2. refers to metadata of image file which monitor can ignor but
tell the printer what size to print.
Size or quality (sharpness) is for calculated size or dimension when viewed close up which means photo is small even like 12R size where the eye can see fine details but when a print is large eg. poster size it will be viewed from farther distance so resolution is not that critical and anyway it's printed with less resolution or DPI.On the contrary insisting a 300 DPi print of the same file size or resolution will result in a smaller photo/image.
If we use 240 DPI for 12 X 15 (12R print) it will give 2880x3600 dots ( 2880/240= 12,3600/240=15).
At 300 DPI we get 2880/300= 9.6, 3600/300= 12 so standard size is 10x12?
TS should inspect the image's pixel dimensions and calculate the optimum enlargement.
Actually ask print shop what resolution/DPI they print at or can do custom print. Now this is for best quality when viewed close up but as he mentioned his client want bigger than 12R so at same "resolution" but viewed from further distance and printed with less DPI this same image (12Mp) can be 30x45 inch and still be good. If TS KS can suggest 24X36 in.

See this link:
https://photographyicon.com/enlarge/
The table from the link above: Legend: purple -superb - 200+ PPI
Dark blue -Excellent -150-199 PPI
Light blue- better - 100-149 PPI
Green- good - 80-90 PPI (Quality that your typical
photographer will be happy with.)
Orange- fair -79PPI or less
