Disclaimer: I am not a scholar, but my answer is based on authentic ahaadeeth and scholarly opinion.
The action of your friends is based on the following hadeeth narrated by Ibn Maajah in his Sunnan:
It was narrated from ‘Aishah, the Mother of the Believers, that some people said:
“O Messenger of Allah, some people bring us meat, and we do not know whether the Name of Allah has been mentioned over it or not.” He said: “Say: Bismillah and eat.’ They were new in Islam.
Based on this hadeeth and other ahaadeeth, scholars have said that the general principle with regards to meat slaughtered by the people of the Book can be eaten without getting into the details of the way the animal has been slaughtered. Shaykh Ibn `Uthaymeen (rahimahu Allaahu ta`aalaa) said:
It is not essential to ask about that which was slaughtered by a Muslim or a kitaabi, and how it was slaughtered, and whether the name of Allaah was mentioned over it or not. Rather that should not be done, because that is being obstinate in religious matters.
However, it should be noted that in this situation, these people weren't certain about the way the meat has been slaughtered. If a person is certain that some meat is not halaal, then it is definitely haraam to eat it. Since we know for sure that animals in the western world today are being stunned before getting slaughtered, so they are essentially always getting killed before they get slaughtered, the general principle with regards to meat that's sold in western countries is that it is haraam, as scholars have mentioned. Allaah says in the Qur'aan:
“Forbidden to you (for food) are: Al‑Maitah (the dead animals — cattle — beast not slaughtered), blood, the flesh of swine, and that on which Allaah’s Name has not been mentioned while slaughtering (that which has been slaughtered as a sacrifice for others than Allaah, or has been slaughtered for idols) and that which has been killed by strangling, or by a violent blow, or by a headlong fall, or by the goring of horns — and that which has been (partly) eaten by a wild animal — unless you are able to slaughter it (before its death)” [al-Maa’idah 5:3]
Therefore, the action of your friends is not justifiable and you should refrain from eating any meat at a non-halaal restaurant.
Finally, brother, know that you have no reason whatsoever to envy your Muslim friends for doing something that's haraam. It should be the other way around. Your Muslim friends are the ones that are supposed to envy you for your actions in which you are seeking Allaah's pleasure, since the reward you will get for being an obedient servant of Allaah is definitely far greater than the pleasure of eating a piece of meat. You should be happy to obey your Lord, subhanahu wata`aalaa, and you should advice your friends to also refrain from eating haraam food.
May Allaah make it all easy for you.