How yould you describe someone who is Ranger-tabbed, versus serving in one of the ranger regiments?

Rangers appreciate that you seek to say this right. I suggest the following: "with one of my colleagues, a Ranger-qualified Officer in an Infantry battalion down the road, admitting to me..."

Talking about two different things. Ranger school is a training course, thus a Ranger tab is essentially a qualification badge indicating that the person demonstrated proficiency at Ranger School. Just state that the person is a "graduate of Ranger School." This indicates that they have undergone and passed the rigorous training and testing required to earn the Ranger Tab, demonstrating their proficiency in a variety of military skills and their readiness to serve in demanding tactical and operational environments.

The Ranger Regiment is one of the 'premier' of those demanding operational assignments. Their preparedness to "fight tonight" goes beyond what was learned in Ranger school and beyond what is typical in standard MTOE infantry units. Hence the saying that the Scroll is a way of life.

If you were ever in the Regiment, or worked near them, you would have often heard "the tab is a school, the scroll is a way of life," implying that in contrast to those who did it for 70 days, they do it every day as a job.

In the context of any Infantry officer in any Infantry Bn, they would all be Ranger School graduates, and it would be redundant to mention that they completed Ranger school.
If they are any other basic or special branch in an Infantry Bn, I suppose it would be worth pointing it out if that experience was also germane to the topic.