I try to treat them just as I would a normal customer. My brother only has 1 arm, but he neither demands or receives any special treatment from anyone. He just wants to be treated like any other person.
The deaf customer I had came thru the line with someone who signed for them. I made it a point to look directly at them whenever I spoke, so he could read my lips. He required little help from his interpreter, and both spoke and signed a "thank you" to me with a big smile. I asked his friend how to sign "you're welcome", which he showed me, and I "replied" to him in kind.
Truthfully, they were the most friendly and memorable customers I've had the pleasure of serving.
Oh man.
Usually we--and I think I'm qualified to speak for the deaf in general, although there are exceptions--just walk into a place, don't pay attention to anything other than what we intend to get, get only those things we want to get, go to the counter, pay for the purchase, and smile at the cashier before leaving.
That's it. A quick, short, in-and-out.
Of course, I'm describing how the deaf shop
solo; in the case you described, another person was with him, and that changes the dynamics of the thing; with three people involved, there's more interaction.
The nice thing about us is that we don't pester, annoy, bother, clerks with questions. If something's not there, or we can't find it, we just shrug our shoulders, "Oh well, maybe next time," and that's it.
I shop
solo about half the time; about one-third of the time, I'm with the neighbor's wife, and given that she's an effervescent, bubbly, chatty person, and that there's four little kids along with us, it's something utterly different.