Assuming I am looking at the right thing:
You should construct the iframe page so that it displays properly on mobile to start with. No other method will produce satisfactory results.
Indeed even when you display on desktop the display is broken because you have inserted the iframe into the right column and there is not enough room for it there. All the text on the products overlap and the side menu is squished.
At about 700px and under the iframe alone is starting to break up and should have had media queries in place to re-arrange the display to something more suitable for smaller screens.
A quick fix would be to make the iframe 100% width of your layout at 700px width or less (i.e. make it span all columns). Then wrap a div around the iframe that is 100% width and set overflow:auto so that the user can scroll to the right to see the whole iframe at a reasonable width.
Another quick fix would be to use the transform property and scale the iframe by 50% smaller ( or whatever is necessary). That should still allow the user to zoom the iframe to read it.
I believe you could get the same effect by setting a viewport meta tag in the iframe itself and setting the viewport meta tag width to 700px but I have not tested that and I’m guessing there will be browsers differences,
The best answer is to make the iframe properly responsive at source.