Don has met Litchi once before, but between the time elapsed and his poor human-recognition skills, he doesn't remember her. He also doesn't know that she's friends with his partner. As such, he stands up to greet her as a new acquaintance.

"I'm Donatello, and this is my friend and advisor, Professor Hastings. We're looking for someone who can help us use the imaging equipment."

No need to explain exactly why until he finds out whether Litchi will be that someone: he'd rather not share his reasons with more people than necessary.