A player should absolutely say they forgot... but they need to remember that they forgot something to be able to do that.
Like, the difference between:
Player: "How about we ask... dang, forgot his name, you know, the thieves' guild Celestial expert guy? We met before that mission at Castle Redgrave, when they told us."
GM: "Renald the Radiant?"
Player: "That's the one, thanks."
And:
Player: "Dang, no one in the group know about those Celestials, and none of our allies do either. Guess we'll have to go fish for info somehow. Let's start by the library."
GM: "Well, your PC would remember you met an expert on the topic. Renald the Radiant, of the thieves' guild."
What you don't get is that it's not a question of "forcing the issue". No one is forcing anything.
To continue the example above, if the player goes "Oh yeah, that guy. He could likely answer our questions, but then the thieves' guild would also know we're going to do something related to the Celestials, and them getting involved is even scarier than going into the situation blind. I vote we search for a different source of info", then the GM should just say "fair" and move on (unless the player forgot or misunderstood something else, of course).
No one is forcing anyone or anything.
It's just that sometime you need to remind the players of a path that could be taken, then the PCs wouldn't have forgotten it. What they decide to do afterward is still just as up to them as before.