Can I send someone a PM even if I'm not friends with them?

Title pretty self explanatory. I'm pretty buzzed and just played a game as Illaoi, and absolutely destroyed my opponent in lane. I ended the game with almost 6x his cs, and was up nearly 200 cs and 3 items at one point. I honestly thought he was just trolling, because he was doing really stupid shit in lane and not speaking. I wasn't flaming him really or being toxic, but I was chirping him and when did something dumb like ignite me at nearly full health, I would comment on that in all chat and ask him why. His team said he wasn't talking to them either. Ultimately we won, but when I looked at his profile, I saw that he had just hit level 30 and was in his placements. I feel awful because I thought he was a troll but in reality he's just new, and I'd really like to apologize to him, maybe give him some actual constructive feedback on how to improve. I would hate for someone new to ranked have such a game be there first experience.