Outlook is a free email service provider – and consequently they send out a TON of spam. Not as much as the drive-by gmail users, but a lot.
Send enough or send to their traps and the IP gets listed. It’s a ~48 hour thing with Spamcop so it’s not like it’s permanent.
Your client of a client sounds like a user, not an admin. If an IP ends up on an RBL – it earned its place there. Given how much spam originates from outlook.com servers it surprises me not one bit they show up on Spamcop’s lists every now and then. If someone wants to complain about it I direct them to Microsoft – since it’s on them to implement better due diligence our better outbound scanning (which I don’t even think they do).
Otherwise – An RBL is an RBL. When it lists an IP there’s usually a pretty good reason for it – so let it do its thing. If someone complains you can always choose to whitelist Outlook IPs so that they bypass RBL checks.
Edit: And having now read that reddit thread – It would appear I’m not alone in my opinion of Outlook’s IPs.