Donald Trump was famous back then too and appeared in many films and TV shows back then. His roles were mostly throwaways/jokes/gigs etc.,
Like the one in The Fresh Prince of Bel-Air:
But that's not all, many shows film used his name as joke and throwaway lines too. In Designing Women (“Marriage Most Foul”, 1991):
Julia Sugarbaker’s call to Donald Trump:
Hello? Mr. Trump? I hope I’m not disturbing you. I’m just calling you to say, on behalf of the American public: Mr. Trump, we no longer care who you date, we really don’t. You are no longer obligated to alert the news media every time your pants are on fire because we don’t care. So please feel free to fire all your hacks, flacks and publicists employed for this purpose because — and I repeat — we don’t care! Who am I? Who am I? Well, you’ve never met me, but you can just call me “The Julia”. - source
So he was a rich and famous guy and Friends used it in the same way.that also was a way to make it clear Chandler and Monica were together. From the episode's transcript
Joey: How was your conference?
Chandler: It was terrible. I fought with (Pause) my colleagues y'know, the entire time. Are you kidding with this? (Throws away another Rollo)
Joey: Oh, so your weekend was a total bust?
Chandler: Uh, no, I got to see Donald Trump waiting for an elevator.
Then later in the same episode :
Phoebe: Oh hey, Monica, I heard you saw Donald Trump at your convention.
Monica: Yeah, I saw him waiting for an elevator.
(Joey thinks that sounds familiar, but dismisses the thought.)
It gives chance to Joey to connect dots which he didn't at the moment but he did know that they were together already.
And this was not the only trump reference, they mentioned him again randomly too:
Joey: Guess what job I just got?
Chandler: I don't know, but Donald Trump wants his blue blazer black. (pauses)
Chandler: Blue blazer back. He wants it back.
Rachel: But, you said "black". Why would he want his blue blazer black?
Chandler: Well, you know what I meant.
Monica: No, you messed it up. You're stupid.
Chandler: (changing the subject) So what job did you get, Joe?
This reference also show the actor's improvisation:
In the episode "The One With Phoebe's Uterus", Joey walks into the apartment wearing a blue blazer. Matthew Perry (Chandler), while making a joke, accidentally said "black" instead of "back", but the actors' reactions to the mistake were so funny, they decided to put it in the episode - - tvtropes