I can't find a threading diagram for the 100 but this one is for several other husq models, so could be very close for your friends.
Make sure that the bobbin is going into the bobbin case as per the diagram and is into the tension spring. The bobbin needs to turn anticlockwise when you pull on the thread tail when looking at the bobbin inside the case.
There is also a little springlatch on the bobbin case, you need to hold this to release it from the rotary hook to take out. If this isn't working correctly, it will pop out so you'd need to replace the bobbin case.
Now check the top threading too. ALWAYS raise the pressure foot to open up the tensions. And turn flywheel towards you until needle is at highest position. Now follow the threading diagram ensuring the thread does into each eyelet, then down the needle bar and into the needle eye.
Check tension knob number, should be between 4-6, usually 5.
Raise bottom thread manually through the needle plate by holding top thread tail and turning flywheel towards you through one revolution, catching the bottom thread and pulling it up through the hole in the needle plate.
Make sure that you have a new needle in, flat shank to the back, and the right size for fabric you wish to sew, sharp for wovens, ball point for knits.
Now select your stitch, place fabric under foot, lower pressure foot and while holding both threads behind the foot, start stitching. Hopefully, it will now sew for you.
If you can't get the bottom thread to come up at all then its likely that the timing is out and it will need a service.