OT?, what happens if you put 2-cycle oil in your lawnmower crankcase?

A friend did this his push rotary mower, probably with a B&S engine.
I can't get a good answer how much.

http://oilstore.stores.yahoo.net/penaircool2c.html

It says nothing about viscosity.

Does he have to change the oil?

I thank you and my friend thanks you.