(A) asks, 'Do you think this is a good idea?' (B) says, 'Yes, l do', - but (B) is more on the side of 'no' not 'yes', but (B) tells (A) what they want to hear so (B) doesnt have to talk about 'the idea'. What is (B)s comment called? Is it a lie as its an untruth? Or something different? (B) says its not lying. (A) says it is as its not the truth.
They are probably playing along (with you) just to avoid an argument.
play along (with somebody/something)
to seem to support or be friendly to someone or something
He knew that if he didn't play along with the reporters, they would write unpleasant stories about him.
I don't really like their idea that much but for now, it is probably a good idea simply to play along.
Cambridge Dictionary of American Idioms Copyright © Cambridge University Press 2003. Reproduced with permission.
B is humoring A.
2 humor verb
1 : to soothe or content (someone) by indulgence : to comply with the temperment or inclinations of • The only way to get along with him is to humor him. • I know you don't agree, but just humor me.
They're going with the flow -
To do what other people are doing or to agree with other people because it is the easiest thing to do
I wasn't very keen on the decision but it was easier just to go with the flow.
to accept, comply, or submit tacitly or passively.
When you acquiesce (to someone), you give them what they want even though you would rather not. Which fits your description.