I mean, the app offers encryption of the data, so you’d have to enter a password. And you can encrypt your phone as well. If it gets to a point where you are forced to enter the password, a piece of paper in your drawer is probably not much safer.
It’s really beyond fucked up that this is something people have to think about.
In that case I don’t think you should go. Your son is 14, he will be able to tell that there is bad blood. Maybe sit him down and explain that you would like to go for his sake but think it’s too soon and you are not comfortable around them yet (without throwing blame on them, as difficult as that may be).