You can't just throw in any old oil! It still must be an oil suitable for wet clutch motorcycle use. It is possible to mix most synthetics and conventional oils nowadays, and you can even buy blends, but there is still a risk that the two will be incompatible. Why take a huge chance like this? If you are going that route, I'd recommend adding a readily available 10W-40 synthetic motorcycle oil, meeting the rest of the spec, just to get by for a short time. In addition, until BRP changes its strict oil recommendation, use of anything outside their specs can void the warraty if damage occurs as a result. I would tell your friend to use an oil meeting the specs...and to find another dealer, pronto!
You might make sure your friend checked the oil level properly. The engine must be to full operating temperature, not merely just warmed up to show a few bars on the gauge. Best time is immediately after a ride. Pull the side panel. start the Spyder for at least 30 seconds, shut off the engine, and check the level immediately, screwing in the dipstick completely. Failure to follow this procedure closely will result in oil not showing on the dipstick.
-Scotty