If they own a Wii, then maybe Magical Melody or Animal Parade. Magical Melody is more simple, though it may be a bit too boring. Everything in this game is really easy though, from raising friendship to earning money from farming. I've never played Animal Parade, but I've heard that it's a fan favorite to the series, so it must be good.
If they own a Wii U, then the virtual console has HM64, HM SNES, and both HM GBA mineral town games. None of them cost more than $9.99 here in NA, so their cheap and easy to just boot up your system, and simply play right away. No need to fear the "save juice" or whatever running out on your GBA cartridge this way. I totally reccomend the FOMT games, as they were my first introduction to the HM series. The farming isn't too easy, while also not being too hard at the same time. Many characters develops over time as you raise their friendship, and I was even learning new things from their lives even in year 3+. Heart events and festivals are fantastic in a simple way as well. And although it's not a big detail, you are given somewhere around 14-16 spaces for naming things, rather than the standard 6-space/character limit for naming your kid/animals/farm, etc.
If they own a DS, then probably Grand Bazaar or Tale of Two Towns. GB was simpler, but more boring once you reach the second or third year. TOTT is more complicated, but at least there's more to do and explore. That game is also really slow though.
Even better, if they own a 3DS, then Trio of Towns (unless you live outside of NA right now). If that's not an option, then I'd go with Story of Seasons next. If that's not an option either, then maybe a RF game. I went into the RF series for the first time with RF4, fully expecting it to be a farming simulator, and was not disappointed in the least. It had everything a HM game had and more, which is extremely nice for people like me, who get tired of constantly farming 24/7. Some HM games can get boring after a long time, but the quirky cast of characters is enough to keep some people in tears from laughing. Hell, the one employee I had talked to while I purchased RF4 said that she showed her friend the game, and it took them a month before they put it down, because it was just that good.
I probably did not help narrow down your list in the slightest, but those were my personal favorites on all the systems I own.