Yes, that's the first thing to do: ensure that your image can stand up to that kind of enlargement. If you want the print to be relatively sharp close up, you're going to need a photo taken on at least 4x5 film or an 80 MP digital back!
However, if the client is happy with the print appearing sharp at an appropriate viewing distance - that is, the closest distance an average person would stand to comfortably view the entire picture - then you can get away with a far lower resolution image file. I wrote this a few years back, some of which you probably already know:
http://www.l-camera-forum.com/topic/112654-digital-printing-pixels-resolution-and-resampling/ - the info is still relevant, though inkjet printers are more sophisticated and less picky about ppi today.
I used to make prints using resolutions appropriate to the viewing distance of a print - mainly because most people owned digital cameras with resolutions of up to only about 12 MP back when I wrote that post, in 2009.
The flaw in the viewing distance vs print file resolution approach is that people will go up close to a print even if it's massive! So, today I print only to a size that I know will be reasonably sharp close up. With good resizing software, you can resample a high-quality photo (i.e. pin sharp, no noise, well exposed and post-processed, etc.) by about 125% before it starts to visibly break down (what's acceptable is up to you), and I'd be prepared to use 200 ppi for extremely large prints (my usual is 300 ppi), bearing in mind that not only will this result in some softness seen close up but any artefacts from resampling will be more visible than at 300 ppi.
As I make large gallery prints (up to A1), I now use a 36 MP Nikon D800E to ensure my large prints are sharp when folk stick their noses into them! To give you an idea of the absolute maximum print size I'd consider acceptable from my Nikon:
Long edge = 7360 pixels
Resampled to 125% = 9200 pixels
Print size @ 200 ppi = 9200/200 = 44 inches.
Let's call that 3.5 feet rather than 4 feet, just to be safe by increasing the sharpness a little.
So, as you can see, even with my 36 MP camera, making a 7 foot print that's sharp when close up or even a few feet back is impossible!