That sentence is perfectly correct and not colloquial in the least. It's a standard mixed conditional. Look at it this way. The friend's inability to pack is a constant fact. Some people can't swim; some people can't sing; he can't pack. This fact about him never changes. He can't pack. But in this scenario, we are imagining an unreality in which he "could pack". We're not hypothesising about an unreal event where he packed the car efficiently: we're hypothesising about a world in which he is a person with packing skills. Here's a similar sentence which illustrates the point more clearly: "I can't swim. If I could swim, I'd have gone scuba-diving with my friends yesterday". Does the mixed conditional make more sense in that sentence?

你好! Phil. I do notice you use Chinese which surprised me. I looked at it over and found that it's very helpful. Thanks very much.

Thank you to Phil for pointing out the pack/park inconsistency. I'm not sure how that happened. I could have sworn the question said 'pack' and that it was something about a badly loaded vehicle. Maybe I need new reading glasses?? Too late to edit my answer now. What I told you about the grammar side of things still holds good, fortunately.

你好, Mike, as Suki said, it’s grammatically correct and natural English. I agree with you though, that this is a good example of how language can appear a bit illogical at times. You may want to take a quick look at my mini-article where I give a quick summary of the conditionals, including mixed conditionals: https://www.italki.com/discussion/164505

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