I have a roundcube install which is humming along just fine, under a trivial load (nearly no one uses it, but that's not the point).

I have a friend who has a mail account that he can access using Horde, but he really hates Horde and it is missing at least one feature that he really wants (Horde will not show if a message has an attachment in the message list without making a change server-side which he cannot do). So, question.

o Can I configure my Roundcube so that he can enter 
  his email and password and my Roundcube will see 
  that's not an address on my server and instead 
  connect to the other mail server as an IMAP client?

(I have all the server info and ports and passwords and such)

If not, I can setup a second roundcube install and point a different subdomain at it, but having just the one would be much better for me.

If I do go the second roundcube route, I'd prefer to install it on my Synology instead of on my mail server, but I don't now how to install plugins on the sinology version of roundcube as I don't think I can install composer?

Second, is there a way that I can add highlighting colors to the message list based on the sender address. He used to do this in Squirrelmail (his preferred webmail client, but that is not possible to run anymore).

I haven't really touched my roundcube config since I added a logon-page plugin and occasionally when it stops working 😃