NTA

This Beth person sounds absolutely dreadful. Telling someone that because they didn't stay long enough for her silly arbitrarily defined standards of what constitutes staying at a funeral "long enough" is absolutely awful.

He may very well have been emotionally overwhelmed by the funeral and wanting to grieve on his own. I've personally had to leave the funeral of someone I dearly loved because it was too much to handle. Seeking comfort in being with someone he cares about is only natural.

Beth grieving does not give her justification for lashing out at someone like that. Not only is that just a terrible thing to do but it sounds like she is to be blunt, emotionally immature and possibly dimwitted for not understanding people grieve in different ways.