Citing unpublished figure from an ex colleague
The correct thing to do is to cite them for the figure by name, if they will permit it. List the citation as "private communication". You should also probably indicate the it is "adapted from a private communication..."

You have committed plagiarism if you give the impression that you are the source. But it is easy to avoid with a citation. 

You can probably handle it in acknowledgements if you are clear about what you are acknowledging. Use language similar to the above.