Can someone who is in jail make a medical decision for someone in the hospital?

He'd have to get the permission of the prison authorities.  Unless your aunt had named him as her health care representative, in writing, that's probably a long shot, and maybe so even if she did that.  The problem is that he's a boyfriend, not a husband, which makes a large legal difference, and that prison authorities' job descriptions don't include being great humanitarians.

If you can arrange for your aunt's doctor to accept a collect call, at whatever time the boyfriend has telephone privileges, it might be possible to take care of this within the prison routine.  I wouldn't hope for a temporary release or a visit under guard, although it doesn't usually hurt to ask.