Bottom line, YES, he'll have no problem what-so-ever using the 2C .60 engine.

He does not have to worry about the difference in flight characteristics on his .40 sized Ultra-Stick.

It's not going to be noticable as the extra power from the 2C .60 will offset any torque curve differences in the 4C .70.

At worst he can merely use a bigger lower pitch prop.