All perfect praise be to Allah
Well we are ordered to eat only HALAL and their is no reference needed for this i think. Now, which animal is not slaughtered islamically, is not halal.
Now comes the your friend who say it is halal for us because we don't have any choice, in-fact you have choice to eat vegetables and fish etc, which is halal so this is not an excuse.
I have never heard of anything like this that muslims can eat haram in this kind of situation, so if someone says so, he must present the evidence for this view. Than those evidence will be evaluated as per islami sharia...
Additionally from this fatwa(Ruling on eating meat that is sold in stores in non-Muslim countries):
But if the Muslim is in those non-Muslim countries and it is difficult for him to obtain meat that is slaughtered in the proper shar’i manner, and he is fed up of eating chicken and the like, this does not justify him eating haraam meat and it does not mean he comes under the heading of one who is compelled or forced, according to the consensus of the Muslims. It is essential to pay attention to this matter and warn against carelessness for which there is no reason. This is what seems to me to be the case with regard to this matter which has become widespread. I ask Allah to help the Muslims to do that which is in their best religious and worldly interests and to instil fear of Him and respect for His sacred limits in their hearts and to help them avoid that which goes against His laws.
Also read this Fatwa (Conditions of eating meat slaughtered by the Jews and Christians)
اللہ اعلم بالصواب
Allah knows best