What confuses you? Mortal sins are sins that have dire effects on the soul, that can lead a soul to Hell. Gossip can be a mortal sin.
Perhaps if a friend wants to talk often about the newest trends, the new things they bought, or a desire to go shopping together, that wouldn’t probably be the best for someone. Often people don’t think this would be bad, but I know from experience that it definitely can be. Materialism has a way of putting objects before God and making us “of” the world. Having a friend like this can enchant us into the world of buying.
Partying friends, I know also from experience, make us believe that the world is a party and fun is what life is about. This is also another way of leading us away from God, even if it is unintentionally, it is easy to forget God this way. Drinking to the point of not being able to reason as usual is also a serious sin.
Oversexual friends can lead someone into promiscuity. I have had a friend who would often make sexual jokes in pretty much any situation, bring their sexual lives into conversation often, and ask my advice on sexual issues that go against my views. I am a very non confrontational person so it is hard for me to correct someone. I was very young when I met this person and didn’t register that while I remain silent to their situation, I am also inspiring them to commit more sin. My silence and their suffocating sexual talk softened my views and somehow I found myself practically giving them advice that went against my religious views.
There are other examples I can give you if you’d like. These are just some I came up with that I know from experience caused harm for me.