Current employer may offer dh perm job. But it is the interview in 2 weeks time, that he really wants. What does he say ?

[Oblomov]

Dh has a temporary assignment, in our local town. Suits him fine, whilst he is looking.
His boss wants to talk to him tomorrow about offering him a perm postion.
If the terms are good, what does he say ?
He has a third interview, for the job he really wants in 2 weeks time. Employer already knows this, because he has informed her of this date.

So, how do you phrase these things ?
Does he accept ? tell her he still wnats to go to the other ? Keep quiet ?
He doesn't want to offend, by saying yes I'll take it, but really I want the other - becasue that is offensive - there must be ways to phrase these things.
Other people - how do they juggle/ what do they say ?
Suggestions please.

bump

Keep quiet. If he has to sign a contract forbiding him from working for a rival company put it off. He can accept then resign.

It is not a rival firm avenanap, so that is not a problem.
But she already knows that he is going for this third interview. Does he just say, if asked, that he feels it is "common courteousy to go ", or something similar.

He needs the experience . Can he rearrange it for when he has a day off?