Long time no see

[ziawj2]

I assigned a letter to my student that invites his friend to a dinner party at his home. He was expreceted to write the letter in informal style.
One of my students wrote, “Haven’t seen you for a long time, how are you?” Could I replace it with “Long time no see, how are you?” I think the sencond one is often used in spoken English, so maybe it is more informal than the first one. What is your opnion? Thank you.

[Grumpy]

"Long time no see" is very informal: certainly more so than "Haven't seen you for a long time". It's certainly used in spoken, colloquial English - but you may think that it is a bit too​ informal to use in a written invitation to dinner.

[Tdol]

Also, there's nothing formal about what your student originally wrote.

[Barb_D]

The comma splice bothers me, though. . How are you?

A comma splice isn't informal. It's just wrong.

[lingokid]

I love "long time no see".....it helps absolute beginneers get into the Anglo-Saxon way of informal expressions as well.:)