Idk what to believe my partner or my friends..

Long story. My friends been telling me said he is a player and blah. I don't even care because he and I aren't in a serious relationship. So, he keeps tell me that he is telling the truth and he swore on his mother's grave, does that mean he is telling the truth or he could be lying? He said all my friends are fake and I shouldn't be around them. I've been with him almost a year. I do not want to look like a fool for him. 😞 he know that I truly am in love with him. I just want to know if someone swear on someone's grave, does that mean it's true? This is the first time he mentioned he swore on his mother's grave.