I'm someone who tends to mispronounce more than the general population. I don't mispronounce because I have an accent but for two reasons:
1. Learned in a book
Case one is, I learned a word in a book and since the word isn't often us in speaking language, I never had the chance to learn the "write" pronunciation.
In those case, I don't mind being corrected as long as it is done in a none mocking manner. So, make sure to have a serious look (or a puzzled one) and remove any smile from your face as it could be misinterpreted (and you don't want the person to think that you are mocking them).
Being alone with the person is also preferable. Being corrected in public is not a good place to be in and would cause unnecessary embarrassment.
For the phrasing, I will suggest saying something like:
I have noticed that you pronounce the word X, X'. Is there a reason for that?
When you put this as a question, you don't assume you have the correct pronunciation. Instead, you just note that you don't pronounce a word the same way which means one of you might be wrong.
If the other person doesn't know what to respond to your question (or after they answered), you can explain why you pronounce it the way you do. You don't need to convince the other person that you pronounce it the right way, you just need to make them think about it so that they can do research of there own and decide how they want to pronounce it in the future (knowing that other people don't pronounce it the same way).
2. "Mind Trick"
Especially when I'm tired, the words tend to mix and switch between them in my mind. I think of a meaning but, instead of picking the right word for this meaning, I pick is "neighbor". A word who is relatively similar (they both have the same "color" in my mind) but doesn't mean the same thing.
I have the same problem with the letters "c" and "s" and, whenever I'm going to say one of those out loud, I always have to pause to make sure my mouth will say the right one.
If your acquaintance has the same problem and sometimes use the right word, sometimes don't, don't correct them. They already know they have this problem and telling them that their tongue slipt again won't achieve anything. So, unless you didn't understand what they wanted to say or you want to clarify something, I would advise to not correct them.
But, if you still want to, you can use the same approach than for point one:
I notice you tend to switch words and I was wondering why?
If they weren't aware of the problem before, now they are. And if there already were, you have an opportunity to know this acquaintance better.