Prefill webform with civicrm activity data
Below please find some screenshots to illustrate how to do this:

1. This is a View of Activities. Each Activity has an Update button [that's a Global: Custom Text field -> Text Update; Rewrite results: Output this field as a link -> to the webform URL with case-worker-form?cid2=[contact_id]&amp;aid=[id] and bootstrap classes btn to make it a button] -> the URL that is constructed is case-worker-form?cid2=41218&amp;aid=19153



2. The webform case-worker-form -> receives this data. That form is set up such that cid1 = the logged in user (the social worker); cid2 = the client; aid = activity id of the activity to be updated; 



3. Let's edit the subject e.g. -> 



4. Result:



5. Note the Webform just has Update Existing Activity set to None (as we're matching by aid)


Add on:
I've double checked that this is (still) working on our latest webform civicrm 5.x

root@f4d0afb6c36d:/var/www/html/sites/default/modules/webform_civicrm# git log
commit 76b444bf5192ddf4663a0222540684a230d8e39e
Merge: b54ddfd ef2a518
Author: Karin Gerritsen &lt;KarinG&gt;
Date:   Thu Sep 12 11:24:26 2019 -0600
    Merge pull request #251 from colemanw/rel7
    Use api to retrieve relationship data


Add - on config notes:


ensure your views field is configured such that commas are stripped from cid and aid before passing them into the webform
ensure your webform is configured to allow an activity of the type you want updated -> setting it to -user select- will ensure that