I don't know if I can keep this simple but I can try.
Hemp and marijuana are plants.  CSD and THC are compounds that can be derived from both of these plants.  Both  plants will give you both compounds in different degrees.  Hemp oil will never get you high ,not enough THC, but unless someone purifies it it will contain some small amount of THC and therefore you can test hot for THC from hemp oil.  The modern day test for THC does not tell how much is present.  Where alcohol testing  shows a percentage and indicates a level of intoxication.  A THC test says yes its there or no it isn't.  You body processes THC like it does alcohol it doesn"t build up over time unless you consume faster than you body processes.  The problem is the process is very slow and less linear dissipation than alcohol and a trace amount can show up for a couple weeks or more.  Another fact about THC is in order to get high there has to be heat involved.  If you eat a green marijuana leaf raw nothing will happen.   Somewhere along the line it has to be smoked, boiled or baked to activate the THC. 
I take a 10mg THC gummy every night before bed, it really helps me sleep.  But then it's legal in Oklahoma.  My government service buddies tho don't take any chances.  Your friend could take CBD oil and be as safe as his supplier is reliable.  You can get THC free CBD oil then you have to trust the source.
Sorry I got so windy.
GFY to anyone who needs one.