"Make sure you aren't spotted leaving or doing anything suspicious when in sight of him before you go." That might be too obvious to be advice, but then again he's pretty sure Lan Wangji never had fun as a real teenager and might not know the basics.
Nie Huaisang is an expert on distracting people and getting in the way of things. And it's not like Wei Wuxian had run off after Lan Wangji every day the first time around. That's only his first of several possible plans he's come up with for distracting Wei Wuxian. Huaisang had also spent much of his spare time this week sneaking in Emperor's Smile and other illicit contraband. That would almost certainly work, but would be a hassle to keep restocking the jars every time. Huaisang supposes he could always ask Wei Wuxian to help get more.
"Though, regarding suspicious things I've noticed - Wen Ning is also one of us, isn't he?"