@6gain We do have "Messages by someuser@somehost" menu item in the context menu. So you can see messages, posted and reposted by a User that you found in any other timeline. This works even for users, mentioned in a message: select "Users of message" menu item, see a list of users, and then select "Messages by ..." in the context menu of that User.
Unfortunately, due to current !gnusocial restrictions, it is not possible to request full user's Timeline from a server for a Remote User (not implemented on a server side). In this case in AndStatus you will see only messages by this user that were downloaded earlier in other ways (e.g. as parts of some conversations...)
There is no separate interface to search for a user. The simplest way is to find his message by his username (where he is an author or is mentioned/replied to) and then goto "Messages by thisUser” https://loadaverage.org/attachment/3361396