The last 3 BAD client meetings, lead by the same employee, are really bugging you but you haven't talked to your employee about their poor preparation, sloppy style and general lack of ownership of the relationship. This is bad. For YOU! If you wait any longer, and it happens again, (and it will) you will likely explode in frustration. Setting aside the fact that exploding is unprofessional, the unintended consequences of your hissy fit may in fact permanently ruin your relationship with your employee. There has to be a better way to defuse this situation.
Simply and clearly mention to your employee what you saw. Stick to the facts. Be clear about your expectations, but don't be too aggressive. "Hi Bob, you've been late to the last 3 staff meetings..." Most folks self correct at this point. If they don't...
Repeat the issue but suggest a cure. "Hi Bob, you were late again. Remember I mentioned it last week? You need to try something different. Maybe target 15 minutes before the meeting to get there. Being on time is important. If it happens again...
At this point, you have an established pattern of behavior that isn't changing. So you need to point out the employee is likely facing a punitive option (maybe even nuclear) if they don't correct it. Your tone should be serious. You should be matter of fact. But you should be clear about the consequences as your employee is entitled to clearly understand their circumstances. "Hey Bob, this problem of lateness is not getting fixed. Don't be late for the next staff meeting or you will be dis-invited going forward.