ArtBistro

 I think the best is to use a ply wood instead of canvas , and use weather primer , then out doors paint that resist the harsh weather , and then make sure you use some protective spray and advice your client to hang it in the shades.

Best wishes,

Inspiration

If it's going to be under a covered porch, out of the sun and other direct elements (rain, snow, etc.) You can paint on canvas with no problem.  Gesso front, back and sides with 2 or 3 coats of gesso and then seal it (front, back and sides) with a coat of acrylic paint.  Use artist-grade acrylics (much higher pigment content) and paint away!  Acrylic is plastic, you pretty much can't hurt the stuff.  What the concern is, is that the canvas will shrink or sag affecting the paint that's adhered to it.  But it shouldn't be a problem.  Last I read, 16th century Italy & France didn't have any air conditioning.  Somehow those paintings managed to survive.  Wood panels will shrink and swell as humidity changes, potentially causing the paint to crack.  Be sure to tell this person the following:  "Direct sunlight WILL FADE COLORS.  There's nothing you can do to prevent that (Marine grade varnish will slow this down, but not prevent it)."  and also, I would not guarantee anything hung outside.  Do the piece, it should last just fine...but don't guarantee it and make sure they understand this.