Long Time No See!

Q. How should you handle characters who have been in the show before, but only briefly? Do you need to remind the reader how they fit into everything, ie:

Jake walks over to Ellen, the waitress/one-night stand who is now carrying his baby.

Should I try to find a way to fit it in the dialog?

No, actually, I think what you've done here is the Right Thing. It's concise, it doesn't derail the read or slow it down. Don't put exposition in dialog unless you think the audience won't remember her, either. Clarity and precision are good. Don't be afraid to do anything for clarity so long as it doesn't bounce the reader out of the read.