Unless they're on OKC to look for dates, they have no business being there. The only messages that they could legitimately send on OKC would be to ask someone on a date.
Is this substantially different?
This isn't true. Some look for companionship on OKC. Why they use OKC for it, I have no idea but 100% of the user base isn't strictly for dates/hooking up.
The right way to go about helping out their clients without potentially spamming OKC users would be to message their client on their own site that they have found someone for them on OkCupid. They could then ask their client to sign up to OkCupid to view the account and message the user.
This is bait and switch and also poaching, same as if a realtor goes to another realtor's web site to find prospects and start soliciting them, plus using false pretenses in your case.
We do our best to not spam and inspire trust by being available 24/7 to answer questions and personally interact with users. At the same time, we're constantly trying to grow so that we can provide the best matches. It's hard! Cut us some slack? No? Because it's HackerNews? Yea, I guess that's okay then.