Can one eat at a friend's home who keeps a kosher home but eats at non kosher restraurants?
See Igros Moshe YD 1:54
In case where religious elderly parents live with their children who are not religious and do not keep Kosher, if needed for health, and lacking another alternative, if the parent sees on a constant basis that their children would never introduce non-Kosher to them in order to not cause them pain, then they can be lenient. However, someone healthy, or other people (someone not living in the house?), or someone who has alternatives (it is not a שעת הדחק) should be strict.