No, he was not.

In Disney’s Sleeping Beauty, there is no reference whatsoever to King Stefan having any sort of prior relationship with Maleficent. The reason Maleficent is angry with the king and queen was because they did not invite her to the christening of the princess, treating her with disrespect. She addressed the king in the same formal manner that she does the queen.

Flora: Why, it's Maleficent!

Merryweather: What does she want here?

Fauna: Shhh!

Maleficent: Well, quite a glittering assemblage, King Stefan. Royalty, nobility, the gentry, and, how quaint, even the rabble.

Maleficent: I really felt quite distressed at not receiving an invitation.

Merryweather: You weren't wanted!

Maleficent: Not wa...? Oh dear, what an awkward situation. I had hoped it was merely due to some oversight. Well, in that event I'd best be on my way.

Queen: And you're not offended, Your Excellency?

Maleficent: Why no, your majesty. And to show I bear no ill will, I, too, shall bestow a gift on the child.
_{^{- Disney’s Sleeping Beauty (1959)}}

The christening of the princess was an event for the entire kingdom to take part in no matter their social class. Inviting the entire kingdom regardless of their status while also excluding Maleficent implies either that they actively wished to exclude her or that she did not merit their consideration, either of which would have been highly disrespectful. Her anger was at the royals’ disregard of her.

Narrator: In a faraway land, long ago, there lived a King and his fair Queen. Many years they had longed for a child, and finally their wish was granted. A daughter was born, and they called her Aurora. Yes, they named her after the dawn, for she filled their lives with sunshine. Then a great holiday was proclaimed throughout the land, so that all of high or low estate could pay homage to the infant Princess. And our story begins on that most joyful day...

She only interacted with the king and queen once - at the christening, when she confronted them about her exclusion, which culminated in her cursing Aurora. In her efforts to see her curse through, Maleficent never interacts directly with the king or the queen again. Once her curse is cast, she focuses solely on monitoring her target, Aurora, and removing any obstacles to her curse’s fulfillment.

Any notion of there being a prior relationship between the king and Maleficent is purely an invention of the live action movie. In the original movie, she only seems to know the king and queen as the rulers of the land.