Be tactful. While he is in TS(or whatever chat program you use) start an in-depth conversation on DPS in general. At some point start to talk about whatever class he plays, make sure to get him involved with the subject. Tell him you want his opinion. Link a rotation you think is good for the class, let him say his opinion on it, counter whatever he says is better or whatever, etc. This way it's just a " casual conversation you struck up".