1)
Why did they say "Avi"?
1.
Rashi, Targum Yonasan: This is like 'my master.' Radak - this is like "v'Avi Re'eh Gam Re'eh" (Shmuel I, 24:11).
2)
What was their counsel?
1.
Rashi, Targum Yonasan: Had he told you to do a matter that requires great toil, you would have done so. All the more so, you should do an easy matter!
2.
Malbim: Had he told you to do a matter that requires great toil, you would have done so, because the Navi commanded. All the more so, you should do an easy matter - (a) because it is easy, and (b) this shows his power, to heal via an easy matter!
