Rambam, Hilchot Shechita 4.7:
If we see a Jew from afar that slaughtered an animal and then left, but we don’t know if he knows the laws or does not , it is permissible to eat the slaughtered meat. Similarly, if one tells his messenger to slaughter for him an animal, and he later finds this slaughtered animal, but does not know if his messenger slaughtered it or someone else, it is permissible to eat. Because most people who perform שחיטה are considered to be the experts.
הֲרֵי שֶׁרָאִינוּ יִשְׂרְאֵלִי מֵרָחוֹק שֶׁשָּׁחַט וְהָלַךְ לוֹ וְלֹא יָדַעְנוּ אִם יוֹדֵעַ אִם אֵינוֹ יוֹדֵעַ הֲרֵי זוֹ מֻתֶּרֶת. וְכֵן הָאוֹמֵר לִשְׁלוּחוֹ צֵא וּשְׁחֹט לִי וּמָצָא הַבְּהֵמָה שְׁחוּטָה וְאֵין יָדוּעַ אִם שְׁלוּחוֹ שְׁחָטָהּ אוֹ אַחֵר הֲרֵי זוֹ מֻתֶּרֶת. שֶׁרֹב הַמְּצוּיִין אֵצֶל שְׁחִיטָה מֻמְחִין הֵן:
Most people who perform שחיטה are considered to be the real-deal.
רוב מצויין אצל שחיטה מומחין הן
Also, earlier on 12a, Rambam’s foundation:
א''ר נחמן אמר רב ראה אחד ששחט אם ראהו מתחלה ועד סוף מותר לאכול משחיטתו ואם לאו אסור לאכול משחיטתו היכי דמי אי דידע דגמיר למה לי ראה ואי דידע דלא גמיר פשיטא ואלא דלא ידע אי גמיר אי לא גמיר
Says Rabbi Nachman: Says Rav. If one saw another that slaughtered [an animal], if he saw his slaughter from the beginning until the end, he is permitted to eat the slaughtered [meat]. But if he did not [see from the beginning until the end], he cannot eat it. What do you mean? If he knows the slaughterer is familiar with all the laws, what is the different if he saw him or not? And if he knows that he is not familiar with all the laws, obviously he can eat from it, [because he can see that he slaughter correctly]! So, this case must be referring to when the one watching the slaughterer is unsure if he knows the laws or does not.
The case you’re describing should fit into the above categories. If not, that may be basis to stay away. Nowadays people in this field will probably have such documentation of a completed training or the like. If he seems to be doing all the Jew-stuff he is probably a Jew.
If he turns out to be a reformed convert, which seems to imply that he is not a convert at all, then it was simply a non-Jew who slaughtered an animal (even according to Jewish law) and it is forbidden for a Jew to eat (Rambam, Hilchot Shechita 4.11).