Shabbat Hosts with Indeterminate Kashrut

Dear Rabbi,
Since becoming observant I generally don’t eat a meal prepared by someone who is not shomer Shabbat, the reason being that if they don’t respect Shabbat how can I assume they’re respecting kashrut? That’s led to some uncomfortable moments over the years when I’ve had to turn down invitations. However, I now feel I’m in a particularly awkward situation: for years my wife and I have accepted invitations at another couple’s home for simchas and Sunday brunch. We know them well, they’re friends, they’re vegetarian and buy only kosher produce. We’ve also recently accepted Shabbat invitations. I assumed they had become Shomer Shabbat but I’ve just become aware that they’re not quite. What should we do? I wouldn’t want to lose their friendship.
All the best,
Shabbetai

Hi Shabbetai
These situations are notoriously awkward and pit important values (personal observance vs. interpersonal relations) in tension with one another.
Your policy of accepting invitations at Shomer Shabbat homes is a good one, as strictly speaking one who does not respect Shabbat cannot be relied upon to uphold the laws of kashrut in a conscientious way. Nevertheless,some posekim maintain that if one has good reason to believe that the (putative) hosts are indeed committed to kosher observance and that moreover they would not serve non-kosher food to their mitzvah-observant guests, we may attribute their lack of Shabbat observance to an insufficient appreciation of the severity of the matter, family practice/influence, longstanding habit, etc, rather than a wholesale rejection of the Torah and its teachings.
There is, however, the problem of the permissibility of food prepared and served on Shabbat by one who is ignorant/neglectful of the relevant laws. Nevertheless IMO in this instance you do not need to imperil your friendship over this by cancelling your plans to join them for a Shabbat meal. Better to use it as an opportunity to subtly teach your hosts about hot plate, urn, slow cooker and other Shabbat appliances (if they do not make use of these already; hopefully they do). Even if their Shabbat observance remains flawed in some respects, they can/should uphold the laws relating to the kitchen on Shabbat. In this way they will hopefully progress toward an uncompromising commitment to Shabbat observance in every way.
Good luck and Shabbat Shalom
Rabbi Rashi Simon