I have been trying to get the wife back into bike again as well, especially now that both our boys are really enjoying it too.
Did a few laps on Bobsled, she seem to like that, then the boys wanted to hit Floppy Bunny, the sight change in difficulty put her off for a while.
Got her out again last week on the Richard Juryn trails and into Circuit 8. The further loop on Circuit 8 was just her speed. The closer loop with all the roots was a bit much. She was happy to be out and just walked over what she couldn't ride. Good fun to have her out.
I have to resist the urge to say try this or try that and only give advice if / when she asks. Being the husband / coach doesn't work for many couples. Best to simply enjoy the ride together. Delta Water Shed is another great option for newer riders.
As for the bike, agreed with above, go FS. Any reasonably new bike that fits properly and is not too heavy will work. Make sure it is not a full on XC race bike with a super steep head angle, having a new rider feel like they are going to go OTB on every little descent does not inspire confidence. On the other side your old 40lb freeride bike is likely not advisable either.
Good luck