WalMart and jpeg printing

On my Macintosh, I converted a friend's wedding photo from tiff to jpeg
and burned it to a CD for a friend so she could have WalMart make it
into Christmas cards. I mailed it off to her, and three weeks later she
called and said WalMart did a test print and it "doesn't look right." I
have no idea what she means; her description over the phone didn't help
me at all. (She lives several hours away, so I can't just drop by to see
it.) She said she took it to KMart and Staples also--same problem. While
on the phone I suggested she try opening it on her PC, but she got an
error message ("jpeg file has errors" or something like that). I tried
it in my daughter's Windows 98 PC and I get the same error message. But
it opens fine on my husband's PC at work (with Windows XP). And of
course it opens and prints fine on my Mac. I burned a new CD and went to
my local WalMart to test it. It showed up on the photo machine screen,
but I didn't do a test print. (Am I correct in assuming that if I could
see it on the WalMart screen it would probably print correctly? I've
never used one of those photo machines.) I plan to mail it to her
tomorrow, but I just remembered that I converted to jpeg from a (tiff)
cmyk file. Should I have converted it from the RGB version instead--or
does the color space not matter when converting to jpeg?