Hi YehudahTzvi.
I sympathize with your situation.
Your mom is likely very vulnerable and the need for companionship has made her rationalize that she can manage a companionship with a non-Jew.
I dont know what you can do, but I can only suggest an approach that I recently heard.
This past Motzei Shabbos, in Lakewood, I went to hear Rabbi Avraham Twerski who was speaking for Project Extreme.
Although this is not your situation, as Project Extreme works with teens who go OTD and get involved with everything that gentile teens can get involved in, Rabbi Twerski’s outlook may help here.
Rabbi Twerski spoke, in part, about having a friendly rapport with these individuals, sharing the Torah outlook along with friendly concern, as well as inviting to share in mitzvos. It seems, the teens, too, were looking for companionship first, and would go to where they found it, no matter the cost.
I have to tell you, that being a btguy, in all mixed relationships, no matter how much the non-Jew says they admire or are interested in our Judaism, sooner or later, once the sugar-coated, infatuation stage ends, it always becomes an issue – to them.
In other words, I think this may resolve itself anyway.
I wish you and your mom mitzvos, happiness, and haztlacha!!
It is an understandable problem, but keep involved with letting her know what seems good now, may not end up that way, and she should avoid that before getting too involved.