silver and a source of heat is the short answer.
A propane torch, flux and a commercial solder along with a piece of wire or clamp to hold you pieces in alignment is the long answer. Track of the Wolf or you local harware store/jeweller can provide you with the materials.
A solder joint is/can be very, very strong (IIRC able to withstand 10,000 PSI) A lot of this strength is dependent on a close fit between pieces.
You can easily learn the skill of soldering to complete your Dad's project. Go to a primary source i.e a published book and read up on it. Once you learn the possibilities you will find uses for solder that you never knew you had! I urge the primary source education as I have read some pretty wrong things about solder joints on the internet
My favorite is that you need space in the joint for the solder
That being said if you have a chance at getting good info on the net this is the most likely place.