I completely challenge that; I know many people who have become fully observant in Modern Orthodox communities.
Of course, I hope I didn’t imply that couldn’t happen. I was merely suggesting that if they moved to a community where their current way of living is acceptable, there is a strong likelihood that they will settle into it permanently.
So that the obvious suggestion for a couple like them is the Upper West Side, where many orthodox people eat kosher style outside the house and don’t cover their hair, etc. But if they move to the UWS, they will not be in a community where people are striving to move beyond that.
That is why I suggested a community like Pelham Pkwy, which as you note, is Modern Orthodox, but has more of an out of town feel such that it is both accepting and growing.