If he buys other stuff too, he may get nailed on returning with that eyepiece.
The current limit is $900. Technically this equipment is duty free, so he'd just get hit with the GST if he was paying attention.
10% GST makes it very close to the difference in price of buying domestically.
Edit: Interestingly as well, the limit is actually higher by post than in your baggage. You may be better off in the long run importing directly from someone like OPT and getting them to send it via USPS, if you are determined to import. At current rates, though, that will save you $30 max.
Regards,
Eric