I would not recommend learning to spar for boxing from a friend who "knows" karate.
Karate is not boxing
They are two separate sports, with distinctive styles and techniques. The adaptability of McGregor aside, hands in karate are at best, 60% of the offensive techniques, and of those offensive techniques, maybe 30% of the hand techniques would be applicable to boxing style sparring. Footwork is different, weight balance and shifting is different, even technique setup and flow is different.
Sparring first? Yeah...don't do that
Part of learning to spar is as much defense as it is anything else. Learning how to take a punch with minimal impact/injury, learning how to move, and above all, learning under the tutelage of someone that can correct technique and limit the ferocity of the sparring. Even as a senior BB, I have to be cautious when sparring with junior belts as they have not learned distancing and control. The likelihood that you hit or get hit just "a little harder than you meant" is the difference between walking away and being carried away. Yes, sparring can be that dangerous.
Don't learn to unlearn
The odds that you watch a few videos and spar your buddy and end up with good form is...almost nonexistent. That means that when you do go to an actual gym/studio, you will have to spend time unlearning all the bad habits that you picked up because you didn't know what you were doing.
Don't underestimate a good instructor
No offense to your buddy, but unless he is a certified instructor and a higher rank for a time, the general person that says "Yeah, I know some _____" means that they got to a mid level colored belt and dropped out. Any martial artist that knows anything won't say "Yeah, come on over and we'll spar". This just increases the likelihood that someone is going to get hurt, and hurt badly. You absolutely need someone that can watch, diagnose, correct and teach the correct form from the word go.
Don't do that. Don't be a Darwin award candidate.