I think a lot of people would respond unhappily to being told, even nicely, "You did something racist to a person you are friendly with (because you didn't think things through/look at it from the other person's perspective)." However, also I think the right thing to do is to tell him anyway, and it seems that is your goal.
Telling someone truthfully that their behavior was racist and inappropriate is not inherently mean or unkind. On the other hand, it very likely will cause him to feel bad and experience some discomfort. He may feel guilty and embarrassed, but hopefully learn from that and not take it out on you. He may double down on denying that he did anything wrong, and accuse you of 'calling him a horrible person'. If the latter happens, there is not a lot you can do with it, besides end the conversation and try to revisit it when he calms down (if you feel it would be productive).
Start by picking a time when you're both relaxed and have time to talk. Let your husband know that you want to talk about something that may be uncomfortable, but it is important to you to share something you learned about with him. This is something I practice in my own relationships, and I find that selecting the right time and mood for a talk and giving a warning that a maybe-uncomfortable discussion is about to happen really improves the chances of things going well. My SO and I have less "stupid" arguments that end in someone being upset since we stopped having intense conversations right before bed when tired, or right before work when rushed and distracted.
If you like, you can then mention the article directly, or just say that what happened at the gas station has stayed in your mind and you need to say something about it.
When you want to tell someone that they behaved inappropriately, it's best to focus on the behavior, not the person. So, don't say something like, "It was racist of you to do that!", say something like, "Touching Black women's hair is objectifying." When I have talked to a friend or family member about a behavior that I felt was wrong (whether the offense was to me or to someone else), I have found that focusing on just the action that occurred, rather than the person who took the action or their assumed motivations or character, helped to prevent things from escalating negatively, since it reduces the perception of being a personal attack. It's easier for most people to hear "this is not the right thing to do" than "YOU did something wrong".
You can gently remind him that because of the power structures in play, (he's white, he's a man, he's probably more physically powerful than her, he's a regular customer at her place of work), she might not have felt free to risk offending him by saying no when asked if he could touch her, even if she didn't really want him to. You can appeal to his sense of empathy and relate this to something in his own experience-- if he's ever worked in a service industry job, he's probably personally familiar with putting up with clients/customers behaving poorly towards him, or agreeing to their unreasonable requests, because sticking up for himself could cause problems with his job. In my experience an appeal to empathy and shared experience is very effective when the person in question truly wants to 'do the right thing', but fails to understand why what they did is wrong. I have used this approach when I wanted a friend to understand why something they said, that seemed innocuous to them, was upsetting for me.
You can soften the blow by emphasizing that you understand that he had no intention to harm or offend anyone, and that you know that he would want to know how to do better in the future. If it makes sense to you, you might also remind him of your shared liberal values, and that you appreciate the fact that you are married to someone who is committed to continuously learning how to treat other people well and with respect.
The strategy of pairing constructive criticism with positive observations or praise and focusing on the action/behavior, not the person, is widely recommended in the business world as well as in personal relationships. One description of this skill is found at https://personalexcellence.co/blog/constructive-criticism/, where it states, regarding the use of compliments as a tool to give criticism without offending:
The feedback sandwich method is a popular method of giving
constructive criticism. It is often used in Toastmasters and in the
corporate environment. I refer to the feedback sandwich as PIP, which
stands for Positive-Improvement-Positive.
Regarding focusing on the action, rather than the person or their character, the same article says:
Comment on the issue, not the person. For example, “The clothes are
dirty” and not “You are dirty.” “The report is late” and not “You are
late.”
