Recently a friend gave me Maple Syrup he made 20 years ago after he cleaned out his sugar house. it had been sealed in a metal can and tastes fine and we have been eatting eat. however, he is considering re-packing it to share with friends. Is this ok or should we just donate it to the compost pile?

In my own experience most if not 70-80% of syrups that appeared to be maple syrup on the shelf were in fact corn syrup masquerading. Be sure to read for "100% maple syrup." You'll know by the price tag too.

Maple syrup will grow mold but the mold can be skimmed off the top.
The mold only grows on the side exposed to air. The mold will die and then naturally seal the maple syrup. Protecting the syrup below the mold.

It does not spoil the rest of the maple syrup.

I have had to throw out refrigerated maple syrup a number of times because it spoiled. In some cases, it has spoiled in well under a year. And, in case you were wondering, we keep a clean fridge and have good food hygene habits.

If your food in your fridge is going bad, plug it in and cool down the contents. It has worked for me for over 50 years.

I've had it grow mold in the fridge. Took about 2 years.

just fyi - it might take a really long time before this happens, but i've definitely had pure maple syrup, kept in the refrigerator, ferment.

Maple syrup Liquor, sounds like a Canadian delight.