Acknowledge previous advisor/collaborators for ideas in article
Acknowledging the contribution of someone, however small it is, is seldom a bad idea.

Usually, when I acknowledge the contribution of someone, I send a copy of the paper to acknowledged people before the final submission, informing them of the acknowledgment. In possible critical cases, I ask whether they are ok or not with the formulation of the acknowledgement, specifying by which date I intend to go on with the submission.

I think that the above approach can be used also in your situation, and should there be no reply by the specified date, I would keep the acknowledgment as it is.